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249 lines
6.7 KiB
Markdown
249 lines
6.7 KiB
Markdown
# Delete Data - DELETE
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Now let's delete some data using **SQLModel**.
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## Continue From Previous Code
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As before, we'll continue from where we left off with the previous code.
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{* ./docs_src/tutorial/update/tutorial003_py310.py ln[0] *}
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Remember to remove the `database.db` file before running the examples to get the same results.
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## Delete with SQL
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This `Spider-Youngster` is getting too weird, so let's just delete it.
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But don't worry, we'll reboot it later with a new story. 😅
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Let's see how to delete it with **SQL**:
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```SQL hl_lines="1"
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DELETE
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FROM hero
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WHERE name = "Spider-Youngster"
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```
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This means, more or less:
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> Hey SQL database 👋, I want to `DELETE` rows `FROM` the table called `hero`.
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>
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> Please delete all the rows `WHERE` the value of the column `name` is equal to `"Spider-Youngster"`.
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Remember that when using a `SELECT` statement it has the form:
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```SQL
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SELECT [some stuff here]
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FROM [name of a table here]
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WHERE [some condition here]
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```
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`DELETE` is very similar, and again we use `FROM` to tell the table to work on, and we use `WHERE` to tell the condition to use to match the rows that we want to delete.
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You can try that in **DB Browser for SQLite**:
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<img class="shadow" src="/img/tutorial/delete/image01.png">
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Have in mind that `DELETE` is to delete entire **rows**, not single values in a row.
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If you want to "delete" a single value in a column while **keeping the row**, you would instead **update** the row as explained in the previous chapter, setting the specific value of the column in that row to `NULL` (to `None` in Python).
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Now let's delete with **SQLModel**.
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To get the same results, delete the `database.db` file before running the examples.
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## Read From the Database
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We'll start by selecting the hero `"Spider-Youngster"` that we updated in the previous chapter, this is the one we will delete:
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{* ./docs_src/tutorial/delete/tutorial001_py310.py ln[70:75] hl[72] *}
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As this is a new function `delete_heroes()`, we'll also add it to the `main()` function so that we call it when executing the program from the command line:
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{* ./docs_src/tutorial/delete/tutorial001_py310.py ln[90:98] hl[94] *}
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That will print the same existing hero **Spider-Youngster**:
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<div class="termy">
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```console
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$ python app.py
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// Some boilerplate and previous output omitted 😉
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// The SELECT with WHERE
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INFO Engine BEGIN (implicit)
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INFO Engine SELECT hero.id, hero.name, hero.secret_name, hero.age
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FROM hero
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WHERE hero.name = ?
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INFO Engine [no key 0.00011s] ('Spider-Youngster',)
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// Print the hero as obtained from the database
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Hero: name='Spider-Youngster' secret_name='Pedro Parqueador' age=16 id=2
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```
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</div>
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## Delete the Hero from the Session
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Now, very similar to how we used `session.add()` to add or update new heroes, we can use `session.delete()` to delete the hero from the session:
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{* ./docs_src/tutorial/delete/tutorial001_py310.py ln[70:77] hl[77] *}
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## Commit the Session
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To save the current changes in the session, **commit** it.
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This will save all the changes stored in the **session**, like the deleted hero:
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{* ./docs_src/tutorial/delete/tutorial001_py310.py ln[70:78] hl[78] *}
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The same as we have seen before, `.commit()` will also save anything else that was added to the session. Including updates, or created heroes.
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This commit after deleting the hero will generate this output:
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<div class="termy">
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```console
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$ python app.py
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// Some boilerplate output omitted 😉
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// Previous output omitted 🙈
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// The SQL to update the hero in the database
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INFO Engine DELETE FROM hero WHERE hero.id = ?
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INFO Engine [generated in 0.00020s] (2,)
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INFO Engine COMMIT
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```
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</div>
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## Print the Deleted Object
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Now the hero is deleted from the database.
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If we tried to use `session.refresh()` with it, it would raise an exception, because there's no data in the database for this hero.
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Nevertheless, the object is still available with its data, but now it's not connected to the session and it no longer exists in the database.
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As the object is not connected to the session, it is not marked as "expired", the session doesn't even care much about this object anymore.
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Because of that, the object still contains its attributes with the data in it, so we can print it:
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{* ./docs_src/tutorial/delete/tutorial001_py310.py ln[70:80] hl[80] *}
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This will output:
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<div class="termy">
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```console
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$ python app.py
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// Some boilerplate output omitted 😉
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// Previous output omitted 🙈
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// Print the deleted hero
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Deleted hero: name='Spider-Youngster' secret_name='Pedro Parqueador' age=16 id=2
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```
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</div>
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## Query the Database for the Same Row
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To confirm if it was deleted, now let's query the database again, with the same `"Spider-Youngster"` name:
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{* ./docs_src/tutorial/delete/tutorial001_py310.py ln[70:84] hl[82:84] *}
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Here we are using `results.first()` to get the first object found (in case it found multiple) or `None`, if it didn't find anything.
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If we used `results.one()` instead, it would raise an exception, because it expects exactly one result.
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And because we just deleted that hero, this should not find anything and we should get `None`.
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This will execute some SQL in the database and output:
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<div class="termy">
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```console
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$ python app.py
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// Some boilerplate output omitted 😉
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// Previous output omitted 🙈
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// Automatically start a new transaction
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INFO Engine BEGIN (implicit)
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// SQL to search for the hero
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INFO Engine SELECT hero.id, hero.name, hero.secret_name, hero.age
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FROM hero
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WHERE hero.name = ?
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INFO Engine [no key 0.00013s] ('Spider-Youngster',)
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```
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</div>
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## Confirm the Deletion
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Now let's just confirm that, indeed, no hero was found in the database with that name.
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We'll do it by checking that the "first" item in the `results` is `None`:
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{* ./docs_src/tutorial/delete/tutorial001_py310.py ln[70:87] hl[86:87] *}
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This will output:
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<div class="termy">
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```console
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$ python app.py
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// Some boilerplate output omitted 😉
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// Previous output omitted 🙈
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// Indeed, the hero was deleted 🔥
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There's no hero named Spider-Youngster
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// Cleanup after the with block
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INFO Engine ROLLBACK
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```
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</div>
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## Review the Code
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Now let's review all that code:
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//// tab | Python 3.10+
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```{ .python .annotate hl_lines="70-88" }
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{!./docs_src/tutorial/delete/tutorial002_py310.py!}
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```
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{!./docs_src/tutorial/delete/annotations/en/tutorial002.md!}
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////
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//// tab | Python 3.8+
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```{ .python .annotate hl_lines="72-90" }
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{!./docs_src/tutorial/delete/tutorial002.py!}
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```
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{!./docs_src/tutorial/delete/annotations/en/tutorial002.md!}
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////
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/// tip
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Check out the number bubbles to see what is done by each line of code.
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///
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## Recap
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To delete rows with **SQLModel** you just have to `.delete()` them with the **session**, and then, as always, `.commit()` the session to save the changes to the database. 🔥
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