Implemented \Traversable in ArrayExpression (#15733)

* Implemented \Traversable in ArrayExpression

* Fix punctuation [skip ci]

framework/db/ArrayExpression.php

@@ -7,6 +7,9 @@
 
 namespace yii\db;
 
+use Traversable;
+use yii\base\InvalidConfigException;
+
 /**
  * Class ArrayExpression represents an array SQL expression.
  *
@@ -22,7 +25,7 @@ namespace yii\db;
  * @author Dmytro Naumenko <d.naumenko.a@gmail.com>
  * @since 2.0.14
  */
-class ArrayExpression implements ExpressionInterface, \ArrayAccess, \Countable
+class ArrayExpression implements ExpressionInterface, \ArrayAccess, \Countable, \IteratorAggregate
 {
     /**
      * @var null|string the type of the array elements. Defaults to `null` which means the type is
@@ -33,8 +36,8 @@ class ArrayExpression implements ExpressionInterface, \ArrayAccess, \Countable
      */
     private $type;
     /**
-     * @var array|QueryInterface|mixed the array content. Either represented as an array of values or a [[Query]] that
-     * returns these values. A single value will be considered as an array containing one element.
+     * @var array|QueryInterface the array's content.
+     * In can be represented as an array of values or a [[Query]] that returns these values.
      */
     private $value;
     /**
@@ -167,4 +170,21 @@ class ArrayExpression implements ExpressionInterface, \ArrayAccess, \Countable
     {
         return count($this->value);
     }
+
+    /**
+     * Retrieve an external iterator
+     *
+     * @link http://php.net/manual/en/iteratoraggregate.getiterator.php
+     * @return Traversable An instance of an object implementing <b>Iterator</b> or
+     * <b>Traversable</b>
+     * @since 5.0.0
+     */
+    public function getIterator()
+    {
+        if ($this->getValue() instanceof QueryInterface) {
+            throw new InvalidConfigException('The ArrayExpression class can not be iterated when the value is a QueryInterface object');
+        }
+
+        return new \ArrayIterator($this->getValue());
+    }
 }