allow passing a single Expression object to select() and addSelect()

fixes #9883

framework/CHANGELOG.md

@@ -22,6 +22,7 @@ Yii Framework 2 Change Log
 - Bug #9754: Fixed `yii\web\Request` error when path info is empty (dynasource)
 - Bug #9791: Fixed endless loop on file creation for non-existing device letters on windows (lukos, cebe)
 - Bug #9874: Fixed outputting exception stacktrace in non-debug mode when `Response::FORMAT_RAW` is used (nainoon)
+- Bug #9883: Passing a single `yii\db\Expression` to `Query::select()` or `::addSelect()` was not handled correctly in all cases (cebe)
 - Bug: Fixed generation of canonical URLs for `ViewAction` pages (samdark)
 - Enh #7581: Added ability to specify range using anonymous function in `RangeValidator` (RomeroMsk)
 - Enh #8613: `yii\widgets\FragmentCache` will not store empty content anymore which fixes some problems related to `yii\filters\PageCache` (kidol)

framework/db/Query.php

@@ -397,11 +397,12 @@ class Query extends Component implements QueryInterface
 
     /**
      * Sets the SELECT part of the query.
-     * @param string|array $columns the columns to be selected.
+     * @param string|array|Expression $columns the columns to be selected.
      * Columns can be specified in either a string (e.g. "id, name") or an array (e.g. ['id', 'name']).
      * Columns can be prefixed with table names (e.g. "user.id") and/or contain column aliases (e.g. "user.id AS user_id").
      * The method will automatically quote the column names unless a column contains some parenthesis
-     * (which means the column contains a DB expression).
+     * (which means the column contains a DB expression). A DB expression may also be passed in form of
+     * an [[Expression]] object.
      *
      * Note that if you are selecting an expression like `CONCAT(first_name, ' ', last_name)`, you should
      * use an array to specify the columns. Otherwise, the expression may be incorrectly split into several parts.
@@ -418,7 +419,9 @@ class Query extends Component implements QueryInterface
      */
     public function select($columns, $option = null)
     {
-        if (!is_array($columns)) {
+        if ($columns instanceof Expression) {
+            $columns = [$columns];
+        } elseif (!is_array($columns)) {
             $columns = preg_split('/\s*,\s*/', trim($columns), -1, PREG_SPLIT_NO_EMPTY);
         }
         $this->select = $columns;
@@ -436,13 +439,16 @@ class Query extends Component implements QueryInterface
      * $query->addSelect(["*", "CONCAT(first_name, ' ', last_name) AS full_name"])->one();
      * ```
      *
-     * @param string|array $columns the columns to add to the select.
+     * @param string|array|Expression $columns the columns to add to the select. See [[select()]] for more
+     * details about the format of this parameter.
      * @return $this the query object itself
      * @see select()
      */
     public function addSelect($columns)
     {
-        if (!is_array($columns)) {
+        if ($columns instanceof Expression) {
+            $columns = [$columns];
+        } elseif (!is_array($columns)) {
             $columns = preg_split('/\s*,\s*/', trim($columns), -1, PREG_SPLIT_NO_EMPTY);
         }
         if ($this->select === null) {