yii2/docs/guide/db-active-record.md

Active Record

Note: This section is under development.

Active Record provides an object-oriented interface for accessing and manipulating data stored in databases. An Active Record class is associated with a database table, an Active Record instance corresponds to a row of that table, and an attribute of an Active Record instance represents the value of a particular column in that row. Instead of writing raw SQL statements, you would access Active Record attributes and call Active Record methods to access and manipulate the data stored in database tables.

For example, assume Customer is an Active Record class which is associated with the customer table and name is a column of the customer table. You can write the following code to insert a new row into the customer table:

$customer = new Customer();
$customer->name = 'Qiang';
$customer->save();

The above code is equivalent to using the following raw SQL statement for MySQL, which is less intuitive, more error prone, and may even have compatibility problems if you are using a different kind of database:

$db->createCommand('INSERT INTO `customer` (`name`) VALUES (:name)', [
    ':name' => 'Qiang',
])->execute();

Yii provides the Active Record support for the following relational databases:

• MySQL 4.1 or later: via yii\db\ActiveRecord
• PostgreSQL 7.3 or later: via yii\db\ActiveRecord
• SQLite 2 and 3: via yii\db\ActiveRecord
• Microsoft SQL Server 2008 or later: via yii\db\ActiveRecord
• Oracle: via yii\db\ActiveRecord
• CUBRID 9.3 or later: via yii\db\ActiveRecord (Note that due to a bug in the cubrid PDO extension, quoting of values will not work, so you need CUBRID 9.3 as the client as well as the server)
• Sphinx: via yii\sphinx\ActiveRecord, requires the yii2-sphinx extension
• ElasticSearch: via yii\elasticsearch\ActiveRecord, requires the yii2-elasticsearch extension

Additionally, Yii also supports using Active Record with the following NoSQL databases:

• Redis 2.6.12 or later: via yii\redis\ActiveRecord, requires the yii2-redis extension
• MongoDB 1.3.0 or later: via yii\mongodb\ActiveRecord, requires the yii2-mongodb extension

In this tutorial, we will mainly describe the usage of Active Record for relational databases. However, most content described here are also applicable to Active Record for NoSQL databases.

Declaring Active Record Classes

To get started, declare an Active Record class by extending yii\db\ActiveRecord. Because each Active Record class is associated with a database table, in this class you should override the yii\db\ActiveRecord::tableName() method to specify which table the class is associated with.

In the following example, we declare an Active Record class named Customer for the customer database table.

namespace app\models;

use yii\db\ActiveRecord;

class Customer extends ActiveRecord
{
    const STATUS_INACTIVE = 0;
    const STATUS_ACTIVE = 1;
    
    /**
     * @return string the name of the table associated with this ActiveRecord class.
     */
    public static function tableName()
    {
        return 'customer';
    }
}

Active Record instances are considered as models. For this reason, we usually put Active Record classes under the app\models namespace (or other namespaces for keeping model classes).

Because yii\db\ActiveRecord extends from yii\base\Model, it inherits all model features, such as attributes, validation rules, data serialization, etc.

Connecting to Databases

By default, Active Record uses the db application component as the yii\db\Connection to access and manipulate the database data. As explained in Database Access Objects, you can configure the db component in the application configuration like shown below,

return [
    'components' => [
        'db' => [
            'class' => 'yii\db\Connection',
            'dsn' => 'mysql:host=localhost;dbname=testdb',
            'username' => 'demo',
            'password' => 'demo',
        ],
    ],
];

If you want to use a different database connection other than the db component, you should override the yii\db\ActiveRecord::getDb() method:

class Customer extends ActiveRecord
{
    // ...

    public static function getDb()
    {
        // use the "db2" application component
        return \Yii::$app->db2;  
    }
}

Querying Data

After declaring an Active Record class, you can use it to query data from the corresponding database table. The process usually takes the following three steps:

1. Create a new query object by calling the yii\db\ActiveRecord::find() method;
2. Build the query object by calling query building methods;
3. Call a query method to retrieve data in terms ofActive Record instances.

As you can see, this is very similar to the procedure with query builder. The only difference is that instead of using the new operator to create a query object, you call yii\db\ActiveRecord::find() to return a new query object which is of class yii\db\ActiveQuery.

Below are some examples showing how to use Active Query to query data:

// return a single customer whose ID is 123
// SELECT * FROM `customer` WHERE `id` = 123
$customer = Customer::find()
    ->where(['id' => 123])
    ->one();

// return all active customers and order them by their IDs
// SELECT * FROM `customer` WHERE `status` = 1 ORDER BY `id`
$customers = Customer::find()
    ->where(['status' => Customer::STATUS_ACTIVE])
    ->orderBy('id')
    ->all();

// return the number of active customers
// SELECT COUNT(*) FROM `customer` WHERE `status` = 1
$count = Customer::find()
    ->where(['status' => Customer::STATUS_ACTIVE])
    ->count();

// return all active customers in an array indexed by customer IDs
// SELECT * FROM `customer`
$customers = Customer::find()
    ->indexBy('id')
    ->all();

In the above, $customer is a Customer object while $customers is an array of Customer objects. They are all populated with the data retrieved from the customer table.

Info: Because yii\db\ActiveQuery extends from yii\db\Query, you can use all query building methods and query methods as described in the Section Query Builder.

Because it is a common task to query by primary key values or a set of column values, Yii provides two shortcut methods for this purpose:

• yii\db\ActiveRecord::findOne(): returns a single Active Record instance populated with the first row of the query result.
• yii\db\ActiveRecord::findAll(): returns an array of Active Record instances populated with all query result.

Both methods can take one of the following parameter formats:

• a scalar value: the value is treated as the desired primary key value to be looked for. Yii will determine automatically which column is the primary key column by reading database schema information.
• an array of scalar values: the array is treated as the desired primary key values to be looked for.
• an associative array: the keys are column names and the values are the corresponding desired column values to be looked for. Please refer to Hash Format for more details.

The following code shows how theses methods can be used:

// returns a single customer whose ID is 123
// SELECT * FROM `customer` WHERE `id` = 123
$customer = Customer::findOne(123);

// returns customers whose ID is 100, 101, 123 or 124
// SELECT * FROM `customer` WHERE `id` IN (100, 101, 123, 124)
$customers = Customer::findAll([100, 101, 123, 124]);

// returns an active customer whose ID is 123
// SELECT * FROM `customer` WHERE `id` = 123 AND `status` = 1
$customer = Customer::findOne([
    'id' => 123,
    'status' => Customer::STATUS_ACTIVE,
]);

// returns all inactive customers
// SELECT * FROM `customer` WHERE `status` = 0
$customer = Customer::findAll([
    'status' => Customer::STATUS_INACTIVE,
]);

Note: Neither yii\db\ActiveRecord::findOne() nor yii\db\ActiveQuery::one() will add LIMIT 1 to the generated SQL statement. If your query may return many rows of data, you should call limit(1) explicitly to improve the performance, e.g., Customer::find()->limit(1)->one().

Besides using query building methods, you can also write raw SQLs to query data and populate the results into Active Record objects. You can do so by calling the yii\db\ActiveRecord::queryBySql() method:

// returns all inactive customers
$sql = 'SELECT * FROM customer WHERE status=:status';
$customers = Customer::findBySql($sql, [':status' => Customer::STATUS_INACTIVE])->all();

Do not call extra query building methods after calling yii\db\ActiveRecord::queryBySql() as they will be ignored.

Accessing Data

As aforementioned, the data brought back from the database are populated into Active Record instances, and each row of the query result corresponds to a single Active Record instance. You can access the column values by accessing the attributes of the Active Record instances, for example,

// "id" and "email" are the names of columns in the "customer" table
$customer = Customer::findOne(123);
$id = $customer->id;
$email = $customer->email;

Note: The Active Record attributes are named after the associated table columns in a case-sensitive manner. Yii automatically defines an attribute in Active Record for every column of the associated table. You should NOT redeclare any of the attributes.

Because Active Record attributes are named after table columns, you may find you are writing PHP code like $customer->first_name, which uses underscores to separate words in attribute names if your table columns are named in this way. If you are concerned about code style consistency, you should rename your table columns accordingly (to use camelCase, for example.)

Data Transformation

It often happens that the data being entered and/or displayed are in a different format from the one used in storing the data in a database. For example, in the database you are storing customers' birthdays as UNIX timestamps (which is not a good design, though), while in most cases you would like to manipulate birthdays as strings in the format of 'YYYY/MM/DD'. To achieve this goal, you can define data transformation methods in the Customer Active Record class like the following:

class Customer extends ActiveRecord
{
    // ...

    public function getBirthdayText()
    {
        return date('Y/m/d', $this->birthday);
    }
    
    public function setBirthdayText($value)
    {
        $this->birthday = strtotime($value);
    }
}

Now in your PHP code, instead of accessing $customer->birthday, you would access $customer->birthdayText, which will allow you to input and display customer birthdays in the format of 'YYYY/MM/DD'.

Retrieving Data in Arrays

While retrieving data in terms of Active Record objects is convenient and flexible, it is not always desirable when you have to bring back a large amount of data due to the big memory footprint. In this case, you can retrieve data using PHP arrays by calling yii\db\ActiveQuery::asArray() before executing a query method:

// return all customers
// each customer is returned as an associative array
$customers = Customer::find()
    ->asArray()
    ->all();

Note: While this method saves memory and improves performance, it is closer to the lower DB abstraction layer and you will lose most of the Active Record features. A very important distinction lies in the data type of the column values. When you return data in Active Record instances, column values will be automatically typecast according to the actual column types; on the other hand when you return data in arrays, column values will be strings (since they are the result of PDO without any processing), regardless their actual column types.

Retrieving Data in Batches

In Query Builder, we have explained that you may use batch query to minimize your memory usage when querying a large amount of data from the database. You may use the same technique in Active Record. For example,

// fetch 10 customers at a time
foreach (Customer::find()->batch(10) as $customers) {
    // $customers is an array of 10 or fewer Customer objects
}

// fetch 10 customers at a time and iterate them one by one
foreach (Customer::find()->each(10) as $customer) {
    // $customer is a Customer object
}

// batch query with eager loading
foreach (Customer::find()->with('orders')->each() as $customer) {
    // $customer is a Customer object
}

Saving Data

Using Active Record, you can easily save data to database by taking the following steps:

1. Prepare an Active Record instance
2. Assign new values to Active Record attributes
3. Call yii\db\ActiveRecord::save() to save the data into database.

For example,

// insert a new row of data
$customer = new Customer();
$customer->name = 'James';
$customer->email = 'james@example.com';
$customer->save();

// update an existing row of data
$customer = Customer::findOne(123);
$customer->email = 'james@newexample.com';
$customer->save();

The yii\db\ActiveRecord::save() method can either insert or update a row of data, depending on the state of the Active Record instance. If the instance is newly created via the new operator, calling yii\db\ActiveRecord::save() will cause insertion of a new row; If the instance is the result of a query method, calling yii\db\ActiveRecord::save() will update the row associated with the instance.

You can differentiate the two states of an Active Record instance by checking its yii\db\ActiveRecord::isNewRecord property value. This property is also used by yii\db\ActiveRecord::save() internally as follows:

public function save($runValidation = true, $attributeNames = null)
{
    if ($this->getIsNewRecord()) {
        return $this->insert($runValidation, $attributeNames);
    } else {
        return $this->update($runValidation, $attributeNames) !== false;
    }
}

Tip: You can call yii\db\ActiveRecord::insert() or yii\db\ActiveRecord::update() directly to insert or update a row.

Data Validation

Because yii\db\ActiveRecord extends from yii\base\Model, it shares the same data validation feature. You can declare validation rules by overriding the yii\db\ActiveRecord::rules() method and perform data validation by calling the yii\db\ActiveRecord::validate() method.

When you call yii\db\ActiveRecord::save(), by default it will call yii\db\ActiveRecord::validate() automatically. Only when the validation passes, will it actually save the data; otherwise it will simply return false, and you can check the yii\db\ActiveRecord::errors property to retrieve the validation error messages.

Tip: If you are certain that your data do not need validation (e.g., the data comes from trustable sources), you can call save(false) to skip the validation.

Massive Assignment

Like normal models, Active Record instances also enjoy the massive assignment feature. Using this feature, you can assign values to multiple attributes of an Active Record instance in a single PHP statement, like shown below. Do remember that only safe attributes can be massively assigned, though.

$values = [
    'name' => 'James',
    'email' => 'james@example.com',
];

$customer = new Customer();

$customer->attributes = $values;
$customer->save();

Updating Counters

It is a common task to increment or decrement a column in a database table. We call such columns as counter columns. You can use yii\db\ActiveRecord::updateCounters() to update one or multiple counter columns. For example,

$post = Post::findOne(100);

// UPDATE `post` SET `view_count` = `view_count` + 1 WHERE `id` = 100
$post->updateCounters(['view_count' => 1]);

Note: If you use yii\db\ActiveRecord::save() to update a counter column, you may end up with inaccurate result, because it is likely the same counter is being saved by multiple requests which read and write the same counter value.

Dirty Attributes

When you call yii\db\ActiveRecord::save() to save an Active Record instance, only dirty attributes are being saved. An attribute is considered dirty if its value has been modified since it was loaded from DB or saved to DB most recently. Note that data validation will be performed regardless if the Active Record instance has dirty attributes or not.

Active Record automatically maintains the list of dirty attributes. It does so by maintaining an older version of the attribute values and comparing them with the latest one. You can call yii\db\ActiveRecord::getDirtyAttributes() to get the attributes that are currently dirty. You can also call yii\db\ActiveRecord::markAttributeDirty() to explicitly mark an attribute as dirty.

If you are interested in the attribute values prior to their most recent modification, you may call yii\db\ActiveRecord::getOldAttributes() or yii\db\ActiveRecord::getOldAttribute().

Default Attribute Values

Some of your table columns may have default values defined in the database. Sometimes, you may want to pre-populate your Web form for an Active Record instance with these default values. To avoid writing the same default values again, you can call yii\db\ActiveRecord::loadDefaultValues() to populate the DB-defined default values into the corresponding Active Record attributes:

$customer = new Customer();
$customer->loadDefaultValues();
// $customer->xyz will be assigned the default value declared when defining the "xyz" column

Updating Multiple Rows

The methods described above all work on individual Active Record instances, causing inserting or updating of individual table rows. To update multiple rows simultaneously, you should call yii\db\ActiveRecord::updateAll(), instead, which is a static method.

// UPDATE `customer` SET `status` = 1 WHERE `email` LIKE `%@example.com`
Customer::updateAll(['status' => Customer::STATUS_ACTIVE], ['like', 'email', '@example.com']);

Similarly, you can call yii\db\ActiveRecord::updateAllCounters() to update counter columns of multiple rows at the same time.

// UPDATE `customer` SET `age` = `age` + 1
Customer::updateAllCounters(['age' => 1]);

Deleting Data

To delete a single row of data, first retrieve the Active Record instance corresponding to that row and then call the yii\db\ActiveRecord::delete() method.

$customer = Customer::findOne(123);
$customer->delete();

You can call yii\db\ActiveRecord::deleteAll() to delete multiple or all rows of data. For example,

Customer::deleteAll(['status' => Customer::STATUS_INACTIVE]);

Note: Be very careful when calling yii\db\ActiveRecord::deleteAll() because it may totally erase all data from your table if you make a mistake in specifying the condition.

Active Record Life Cycles

It is important to understand the life cycles of Active Record when it is used for different purposes. During each life cycle, a certain sequence of methods will be invoked, and you can override these methods to get a chance to customize the life cycle. You can also respond to certain Active Record events triggered during a life cycle to inject your custom code. These events are especially useful when you are developing Active Record behaviors which need to customize Active Record life cycles.

In the following, we will summarize various Active Record life cycles and the methods/events that are involved in the life cycles.

New Instance Life Cycle

When creating a new Active Record instance via the new operator, the following life cycle will happen:

1. class constructor;
2. yii\db\ActiveRecord::init(): triggers an yii\db\ActiveRecord::EVENT_INIT event.

Querying Data Life Cycle

When querying data through one of the querying methods, each newly populated Active Record will undergo the following life cycle:

1. class constructor.
2. yii\db\ActiveRecord::init(): triggers an yii\db\ActiveRecord::EVENT_INIT event.
3. yii\db\ActiveRecord::afterFind(): triggers an yii\db\ActiveRecord::EVENT_AFTER_FIND event.

Saving Data Life Cycle

When calling yii\db\ActiveRecord::save() to insert or update an Active Record instance, the following life cycle will happen:

1. yii\db\ActiveRecord::beforeValidate(): triggers an yii\db\ActiveRecord::EVENT_BEFORE_VALIDATE event. If the method returns false or yii\base\ModelEvent::isValid is false, the rest of the steps will be skipped.
2. Performs data validation. If data validation fails, the steps after Step 3 will be skipped.
3. yii\db\ActiveRecord::afterValidate(): triggers an yii\db\ActiveRecord::EVENT_AFTER_VALIDATE event.
4. yii\db\ActiveRecord::beforeSave(): triggers an yii\db\ActiveRecord::EVENT_BEFORE_INSERT or yii\db\ActiveRecord::EVENT_BEFORE_UPDATE event. If the method returns false or yii\base\ModelEvent::isValid is false, the rest of the steps will be skipped.
5. Performs the actual data insertion or updating;
6. yii\db\ActiveRecord::afterSave(): triggers an yii\db\ActiveRecord::EVENT_AFTER_INSERT or yii\db\ActiveRecord::EVENT_AFTER_UPDATE event.

Deleting Data Life Cycle

When calling yii\db\ActiveRecord::delete() to delete an Active Record instance, the following life cycle will happen:

1. yii\db\ActiveRecord::beforeDelete(): triggers an yii\db\ActiveRecord::EVENT_BEFORE_DELETE event. If the method returns false or yii\base\ModelEvent::isValid is false, the rest of the steps will be skipped.
2. perform the actual data deletion
3. yii\db\ActiveRecord::afterDelete(): triggers an yii\db\ActiveRecord::EVENT_AFTER_DELETE event.

Note: Calling any of the following methods will NOT initiate any of the above life cycles:

• yii\db\ActiveRecord::updateAll()
• yii\db\ActiveRecord::deleteAll()
• yii\db\ActiveRecord::updateCounters()
• yii\db\ActiveRecord::updateAllCounters()

Transactional Operations

There are two ways of dealing with transactions while working with Active Record. First way is doing everything manually as described in the "transactions" section of "Database basics". Another way is to implement the transactions method where you can specify which operations are to be wrapped into transactions on a per model scenario:

class Post extends \yii\db\ActiveRecord
{
    public function transactions()
    {
        return [
            'admin' => self::OP_INSERT,
            'api' => self::OP_INSERT | self::OP_UPDATE | self::OP_DELETE,
            // the above is equivalent to the following:
            // 'api' => self::OP_ALL,
        ];
    }
}

In the above admin and api are model scenarios and the constants starting with OP_ are operations that should be wrapped in transactions for these scenarios. Supported operations are OP_INSERT, OP_UPDATE and OP_DELETE. OP_ALL stands for all three.

Such automatic transactions are especially useful if you're doing additional database changes in beforeSave, afterSave, beforeDelete, afterDelete and want to be sure that both succeeded before they are saved.

Optimistic Locks

Optimistic locking is a way to prevent conflicts that may occur when a single row of data is being updated by multiple users. For example, both user A and user B are editing the same wiki article at the same time. After user A saves his edits, user B clicks on the "Save" button in an attempt to save his edits as well. Because user B was actually working on an outdated version of the article, it would be desirable to have a way to prevent him from saving the article and show him some hint message.

Optimistic locking solves the above problem by using a column to record the version number of each row. When a row is being saved with an outdated version number, a yii\db\StaleObjectException exception will be thrown, which prevents the row from being saved. Optimistic locking is only supported when you update or delete an existing row of data using yii\db\ActiveRecord::update() or yii\db\ActiveRecord::delete(), respectively.

To use optimistic locking,

1. Create a column to store the version number of each row. The column type should be BIGINT DEFAULT 0. Override the optimisticLock() method to return the name of this column.
2. In the Web form that collects the user input, add a hidden field that stores the lock version of the record being updated.
3. In the controller action that does the data updating, try to catch the \yii\db\StaleObjectException and implement necessary business logic (e.g. merging the changes, prompting staled data) to resolve the conflict.

Working with Relational Data

You can use ActiveRecord to also query a table's relational data (i.e., selection of data from Table A can also pull in related data from Table B). Thanks to ActiveRecord, the relational data returned can be accessed like a property of the ActiveRecord object associated with the primary table.

For example, with an appropriate relation declaration, by accessing $customer->orders you may obtain an array of Order objects which represent the orders placed by the specified customer.

To declare a relation, define a getter method which returns an yii\db\ActiveQuery object that has relation information about the relation context and thus will only query for related records. For example,

class Customer extends \yii\db\ActiveRecord
{
    public function getOrders()
    {
        // Customer has_many Order via Order.customer_id -> id
        return $this->hasMany(Order::className(), ['customer_id' => 'id']);
    }
}

class Order extends \yii\db\ActiveRecord
{
    public function getCustomer()
    {
        // Order has_one Customer via Customer.id -> customer_id
        return $this->hasOne(Customer::className(), ['id' => 'customer_id']);
    }
}

The methods yii\db\ActiveRecord::hasMany() and yii\db\ActiveRecord::hasOne() used in the above are used to model the many-one relationship and one-one relationship in a relational database. For example, a customer has many orders, and an order has one customer. Both methods take two parameters and return an yii\db\ActiveQuery object:

• $class: the name of the class of the related model(s). This should be a fully qualified class name.
• $link: the association between columns from the two tables. This should be given as an array. The keys of the array are the names of the columns from the table associated with $class, while the values of the array are the names of the columns from the declaring class. It is a good practice to define relationships based on table foreign keys.

After declaring relations, getting relational data is as easy as accessing a component property that is defined by the corresponding getter method:

// get the orders of a customer
$customer = Customer::findOne(1);
$orders = $customer->orders;  // $orders is an array of Order objects

Behind the scenes, the above code executes the following two SQL queries, one for each line of code:

SELECT * FROM customer WHERE id=1;
SELECT * FROM order WHERE customer_id=1;

Tip: If you access the expression $customer->orders again, it will not perform the second SQL query again. The SQL query is only performed the first time when this expression is accessed. Any further accesses will only return the previously fetched results that are cached internally. If you want to re-query the relational data, simply unset the existing expression first: unset($customer->orders);.

Sometimes, you may want to pass parameters to a relational query. For example, instead of returning all orders of a customer, you may want to return only big orders whose subtotal exceeds a specified amount. To do so, declare a bigOrders relation with the following getter method:

class Customer extends \yii\db\ActiveRecord
{
    public function getBigOrders($threshold = 100)
    {
        return $this->hasMany(Order::className(), ['customer_id' => 'id'])
            ->where('subtotal > :threshold', [':threshold' => $threshold])
            ->orderBy('id');
    }
}

Remember that hasMany() returns an yii\db\ActiveQuery object which allows you to customize the query by calling the methods of yii\db\ActiveQuery.

With the above declaration, if you access $customer->bigOrders, it will only return the orders whose subtotal is greater than 100. To specify a different threshold value, use the following code:

$orders = $customer->getBigOrders(200)->all();

Note: A relation method returns an instance of yii\db\ActiveQuery. If you access the relation like an attribute (i.e. a class property), the return value will be the query result of the relation, which could be an instance of yii\db\ActiveRecord, an array of that, or null, depending on the multiplicity of the relation. For example, $customer->getOrders() returns an ActiveQuery instance, while $customer->orders returns an array of Order objects (or an empty array if the query results in nothing).

Relations with Junction Table

Sometimes, two tables are related together via an intermediary table called a junction table. To declare such relations, we can customize the yii\db\ActiveQuery object by calling its yii\db\ActiveQuery::via() or yii\db\ActiveQuery::viaTable() method.

For example, if table order and table item are related via the junction table order_item, we can declare the items relation in the Order class like the following:

class Order extends \yii\db\ActiveRecord
{
    public function getItems()
    {
        return $this->hasMany(Item::className(), ['id' => 'item_id'])
            ->viaTable('order_item', ['order_id' => 'id']);
    }
}

The yii\db\ActiveQuery::via() method is similar to yii\db\ActiveQuery::viaTable() except that the first parameter of yii\db\ActiveQuery::via() takes a relation name declared in the ActiveRecord class instead of the junction table name. For example, the above items relation can be equivalently declared as follows:

class Order extends \yii\db\ActiveRecord
{
    public function getOrderItems()
    {
        return $this->hasMany(OrderItem::className(), ['order_id' => 'id']);
    }

    public function getItems()
    {
        return $this->hasMany(Item::className(), ['id' => 'item_id'])
            ->via('orderItems');
    }
}

Lazy and Eager Loading

As described earlier, when you access the related objects for the first time, ActiveRecord will perform a DB query to retrieve the corresponding data and populate it into the related objects. No query will be performed if you access the same related objects again. We call this lazy loading. For example,

// SQL executed: SELECT * FROM customer WHERE id=1
$customer = Customer::findOne(1);
// SQL executed: SELECT * FROM order WHERE customer_id=1
$orders = $customer->orders;
// no SQL executed
$orders2 = $customer->orders;

Lazy loading is very convenient to use. However, it may suffer from a performance issue in the following scenario:

// SQL executed: SELECT * FROM customer LIMIT 100
$customers = Customer::find()->limit(100)->all();

foreach ($customers as $customer) {
    // SQL executed: SELECT * FROM order WHERE customer_id=...
    $orders = $customer->orders;
    // ...handle $orders...
}

How many SQL queries will be performed in the above code, assuming there are more than 100 customers in the database? 101! The first SQL query brings back 100 customers. Then for each customer, a SQL query is performed to bring back the orders of that customer.

To solve the above performance problem, you can use the so-called eager loading approach by calling yii\db\ActiveQuery::with():

// SQL executed: SELECT * FROM customer LIMIT 100;
//               SELECT * FROM orders WHERE customer_id IN (1,2,...)
$customers = Customer::find()->limit(100)
    ->with('orders')->all();

foreach ($customers as $customer) {
    // no SQL executed
    $orders = $customer->orders;
    // ...handle $orders...
}

As you can see, only two SQL queries are needed for the same task!

Info: In general, if you are eager loading N relations among which M relations are defined with via() or viaTable(), a total number of 1+M+N SQL queries will be performed: one query to bring back the rows for the primary table, one for each of the M junction tables corresponding to the via() or viaTable() calls, and one for each of the N related tables.

Note: When you are customizing select() with eager loading, make sure you include the columns that link the related models. Otherwise, the related models will not be loaded. For example,

$orders = Order::find()->select(['id', 'amount'])->with('customer')->all();
// $orders[0]->customer is always null. To fix the problem, you should do the following:
$orders = Order::find()->select(['id', 'amount', 'customer_id'])->with('customer')->all();

Sometimes, you may want to customize the relational queries on the fly. This can be done for both lazy loading and eager loading. For example,

$customer = Customer::findOne(1);
// lazy loading: SELECT * FROM order WHERE customer_id=1 AND subtotal>100
$orders = $customer->getOrders()->where('subtotal>100')->all();

// eager loading: SELECT * FROM customer LIMIT 100
//                SELECT * FROM order WHERE customer_id IN (1,2,...) AND subtotal>100
$customers = Customer::find()->limit(100)->with([
    'orders' => function($query) {
        $query->andWhere('subtotal>100');
    },
])->all();

Inverse Relations

Relations can often be defined in pairs. For example, Customer may have a relation named orders while Order may have a relation named customer:

class Customer extends ActiveRecord
{
    ....
    public function getOrders()
    {
        return $this->hasMany(Order::className(), ['customer_id' => 'id']);
    }
}

class Order extends ActiveRecord
{
    ....
    public function getCustomer()
    {
        return $this->hasOne(Customer::className(), ['id' => 'customer_id']);
    }
}

If we perform the following query, we would find that the customer of an order is not the same customer object that finds those orders, and accessing customer->orders will trigger one SQL execution while accessing the customer of an order will trigger another SQL execution:

// SELECT * FROM customer WHERE id=1
$customer = Customer::findOne(1);
// echoes "not equal"
// SELECT * FROM order WHERE customer_id=1
// SELECT * FROM customer WHERE id=1
if ($customer->orders[0]->customer === $customer) {
    echo 'equal';
} else {
    echo 'not equal';
}

To avoid the redundant execution of the last SQL statement, we could declare the inverse relations for the customer and the orders relations by calling the yii\db\ActiveQuery::inverseOf() method, like the following:

class Customer extends ActiveRecord
{
    ....
    public function getOrders()
    {
        return $this->hasMany(Order::className(), ['customer_id' => 'id'])->inverseOf('customer');
    }
}

Now if we execute the same query as shown above, we would get:

// SELECT * FROM customer WHERE id=1
$customer = Customer::findOne(1);
// echoes "equal"
// SELECT * FROM order WHERE customer_id=1
if ($customer->orders[0]->customer === $customer) {
    echo 'equal';
} else {
    echo 'not equal';
}

In the above, we have shown how to use inverse relations in lazy loading. Inverse relations also apply in eager loading:

// SELECT * FROM customer
// SELECT * FROM order WHERE customer_id IN (1, 2, ...)
$customers = Customer::find()->with('orders')->all();
// echoes "equal"
if ($customers[0]->orders[0]->customer === $customers[0]) {
    echo 'equal';
} else {
    echo 'not equal';
}

Note: Inverse relation cannot be defined with a relation that involves pivoting tables. That is, if your relation is defined with yii\db\ActiveQuery::via() or yii\db\ActiveQuery::viaTable(), you cannot call yii\db\ActiveQuery::inverseOf() further.

Joining with Relations

When working with relational databases, a common task is to join multiple tables and apply various query conditions and parameters to the JOIN SQL statement. Instead of calling yii\db\ActiveQuery::join() explicitly to build up the JOIN query, you may reuse the existing relation definitions and call yii\db\ActiveQuery::joinWith() to achieve this goal. For example,

// find all orders and sort the orders by the customer id and the order id. also eager loading "customer"
$orders = Order::find()->joinWith('customer')->orderBy('customer.id, order.id')->all();
// find all orders that contain books, and eager loading "books"
$orders = Order::find()->innerJoinWith('books')->all();

In the above, the method yii\db\ActiveQuery::innerJoinWith() is a shortcut to yii\db\ActiveQuery::joinWith() with the join type set as INNER JOIN.

You may join with one or multiple relations; you may apply query conditions to the relations on-the-fly; and you may also join with sub-relations. For example,

// join with multiple relations
// find the orders that contain books and were placed by customers who registered within the past 24 hours
$orders = Order::find()->innerJoinWith([
    'books',
    'customer' => function ($query) {
        $query->where('customer.created_at > ' . (time() - 24 * 3600));
    }
])->all();
// join with sub-relations: join with books and books' authors
$orders = Order::find()->joinWith('books.author')->all();

Behind the scenes, Yii will first execute a JOIN SQL statement to bring back the primary models satisfying the conditions applied to the JOIN SQL. It will then execute a query for each relation and populate the corresponding related records.

The difference between yii\db\ActiveQuery::joinWith() and yii\db\ActiveQuery::with() is that the former joins the tables for the primary model class and the related model classes to retrieve the primary models, while the latter just queries against the table for the primary model class to retrieve the primary models.

Because of this difference, you may apply query conditions that are only available to a JOIN SQL statement. For example, you may filter the primary models by the conditions on the related models, like the example above. You may also sort the primary models using columns from the related tables.

When using yii\db\ActiveQuery::joinWith(), you are responsible to disambiguate column names. In the above examples, we use item.id and order.id to disambiguate the id column references because both of the order table and the item table contain a column named id.

By default, when you join with a relation, the relation will also be eagerly loaded. You may change this behavior by passing the $eagerLoading parameter which specifies whether to eager load the specified relations.

And also by default, yii\db\ActiveQuery::joinWith() uses LEFT JOIN to join the related tables. You may pass it with the $joinType parameter to customize the join type. As a shortcut to the INNER JOIN type, you may use yii\db\ActiveQuery::innerJoinWith().

Below are some more examples,

// find all orders that contain books, but do not eager load "books".
$orders = Order::find()->innerJoinWith('books', false)->all();
// which is equivalent to the above
$orders = Order::find()->joinWith('books', false, 'INNER JOIN')->all();

Sometimes when joining two tables, you may need to specify some extra condition in the ON part of the JOIN query. This can be done by calling the yii\db\ActiveQuery::onCondition() method like the following:

class User extends ActiveRecord
{
    public function getBooks()
    {
        return $this->hasMany(Item::className(), ['owner_id' => 'id'])->onCondition(['category_id' => 1]);
    }
}

In the above, the yii\db\ActiveRecord::hasMany() method returns an yii\db\ActiveQuery instance, upon which yii\db\ActiveQuery::onCondition() is called to specify that only items whose category_id is 1 should be returned.

When you perform a query using yii\db\ActiveQuery::joinWith(), the ON condition will be put in the ON part of the corresponding JOIN query. For example,

// SELECT user.* FROM user LEFT JOIN item ON item.owner_id=user.id AND category_id=1
// SELECT * FROM item WHERE owner_id IN (...) AND category_id=1
$users = User::find()->joinWith('books')->all();

Note that if you use eager loading via yii\db\ActiveQuery::with() or lazy loading, the on-condition will be put in the WHERE part of the corresponding SQL statement, because there is no JOIN query involved. For example,

// SELECT * FROM user WHERE id=10
$user = User::findOne(10);
// SELECT * FROM item WHERE owner_id=10 AND category_id=1
$books = $user->books;

Working with Relationships

ActiveRecord provides the following two methods for establishing and breaking a relationship between two ActiveRecord objects:

• yii\db\ActiveRecord::link()
• yii\db\ActiveRecord::unlink()

For example, given a customer and a new order, we can use the following code to make the order owned by the customer:

$customer = Customer::findOne(1);
$order = new Order();
$order->subtotal = 100;
$customer->link('orders', $order);

The yii\db\ActiveRecord::link() call above will set the customer_id of the order to be the primary key value of $customer and then call yii\db\ActiveRecord::save() to save the order into the database.

Cross-DBMS Relations

ActiveRecord allows you to establish relationships between entities from different DBMS. For example: between a relational database table and MongoDB collection. Such a relation does not require any special code:

// Relational database Active Record
class Customer extends \yii\db\ActiveRecord
{
    public static function tableName()
    {
        return 'customer';
    }

    public function getComments()
    {
        // Customer, stored in relational database, has many Comments, stored in MongoDB collection:
        return $this->hasMany(Comment::className(), ['customer_id' => 'id']);
    }
}

// MongoDb Active Record
class Comment extends \yii\mongodb\ActiveRecord
{
    public static function collectionName()
    {
        return 'comment';
    }

    public function getCustomer()
    {
        // Comment, stored in MongoDB collection, has one Customer, stored in relational database:
        return $this->hasOne(Customer::className(), ['id' => 'customer_id']);
    }
}

All Active Record features like eager and lazy loading, establishing and breaking a relationship and so on, are available for cross-DBMS relations.

Note: do not forget Active Record solutions for different DBMS may have specific methods and features, which may not be applied for cross-DBMS relations. For example: usage of yii\db\ActiveQuery::joinWith() will obviously not work with relation to the MongoDB collection.

Scopes

When you call yii\db\ActiveRecord::find() or yii\db\ActiveRecord::findBySql(), it returns an yii\db\ActiveQuery instance. You may call additional query methods, such as yii\db\ActiveQuery::where(), yii\db\ActiveQuery::orderBy(), to further specify the query conditions.

It is possible that you may want to call the same set of query methods in different places. If this is the case, you should consider defining the so-called scopes. A scope is essentially a method defined in a custom query class that calls a set of query methods to modify the query object. You can then use a scope instead of calling a normal query method.

Two steps are required to define a scope. First, create a custom query class for your model and define the needed scope methods in this class. For example, create a CommentQuery class for the Comment model and define the active() scope method like the following:

namespace app\models;

use yii\db\ActiveQuery;

class CommentQuery extends ActiveQuery
{
    public function active($state = true)
    {
        $this->andWhere(['active' => $state]);
        return $this;
    }
}

Important points are:

1. Class should extend from yii\db\ActiveQuery (or another ActiveQuery such as yii\mongodb\ActiveQuery).
2. A method should be public and should return $this in order to allow method chaining. It may accept parameters.
3. Check yii\db\ActiveQuery methods that are very useful for modifying query conditions.

Second, override yii\db\ActiveRecord::find() to use the custom query class instead of the regular yii\db\ActiveQuery. For the example above, you need to write the following code:

namespace app\models;

use yii\db\ActiveRecord;

class Comment extends ActiveRecord
{
    /**
     * @inheritdoc
     * @return CommentQuery
     */
    public static function find()
    {
        return new CommentQuery(get_called_class());
    }
}

That's it. Now you can use your custom scope methods:

$comments = Comment::find()->active()->all();
$inactiveComments = Comment::find()->active(false)->all();

You can also use scopes when defining relations. For example,

class Post extends \yii\db\ActiveRecord
{
    public function getActiveComments()
    {
        return $this->hasMany(Comment::className(), ['post_id' => 'id'])->active();

    }
}

Or use the scopes on-the-fly when performing a relational query:

$posts = Post::find()->with([
    'comments' => function($q) {
        $q->active();
    }
])->all();

Default Scope

If you used Yii 1.1 before, you may know a concept called default scope. A default scope is a scope that applies to ALL queries. You can define a default scope easily by overriding yii\db\ActiveRecord::find(). For example,

public static function find()
{
    return parent::find()->where(['deleted' => false]);
}

Note that all your queries should then not use yii\db\ActiveQuery::where() but yii\db\ActiveQuery::andWhere() and yii\db\ActiveQuery::orWhere() to not override the default condition.