Fixes #1457: support string representation of AR.

framework/yii/db/BaseActiveRecord.php

@@ -93,6 +93,17 @@ abstract class BaseActiveRecord extends Model implements ActiveRecordInterface
 
 
 	/**
+	 * Returns the string representation of this AR instance.
+	 * The default implementation will return the primary key (or JSON representation of the primary key if it is composite).
+	 * @return string the string representation of this AR instance.
+	 */
+	public function __toString()
+	{
+		$pk = $this->getPrimaryKey();
+		return is_array($pk) ? json_encode($pk) : $pk;
+	}
+
+	/**
 	 * Creates an [[ActiveQuery]] instance for query purpose.
 	 *
 	 * @include @yii/db/ActiveRecord-find.md