diff --git a/extensions/yii/elasticsearch/ActiveQuery.php b/extensions/yii/elasticsearch/ActiveQuery.php
index 2a282d3..d67327c 100644
--- a/extensions/yii/elasticsearch/ActiveQuery.php
+++ b/extensions/yii/elasticsearch/ActiveQuery.php
@@ -92,7 +92,7 @@ class ActiveQuery extends Query implements ActiveQueryInterface
 		}
 		/** @var ActiveRecord $modelClass */
 		$modelClass = $this->modelClass;
-		$pk = $modelClass::primaryKey();
+		$pk = $modelClass::primaryKey()[0];
 		if ($this->asArray && $this->indexBy) {
 			foreach ($result['hits']['hits'] as &$row) {
 				if ($pk === '_id') {
@@ -136,7 +136,7 @@ class ActiveQuery extends Query implements ActiveQueryInterface
 			/** @var ActiveRecord $modelClass */
 			$modelClass = $this->modelClass;
 			$model = $result['_source'];
-			$pk = $modelClass::primaryKey();
+			$pk = $modelClass::primaryKey()[0];
 			if ($pk === '_id') {
 				$model['_id'] = $result['_id'];
 			}
@@ -165,7 +165,7 @@ class ActiveQuery extends Query implements ActiveQueryInterface
 			if ($this->asArray) {
 				/** @var ActiveRecord $modelClass */
 				$modelClass = $this->modelClass;
-				$pk = $modelClass::primaryKey();
+				$pk = $modelClass::primaryKey()[0];
 				foreach($models as $key => $model) {
 					if ($pk === '_id') {
 						$model['_source']['_id'] = $model['_id'];
diff --git a/extensions/yii/elasticsearch/ActiveRecord.php b/extensions/yii/elasticsearch/ActiveRecord.php
index 29d62f3..df50039 100644
--- a/extensions/yii/elasticsearch/ActiveRecord.php
+++ b/extensions/yii/elasticsearch/ActiveRecord.php
@@ -171,7 +171,7 @@ class ActiveRecord extends BaseActiveRecord
 	 */
 	public function setPrimaryKey($value)
 	{
-		$pk = static::primaryKey();
+		$pk = static::primaryKey()[0];
 		if ($this->getIsNewRecord() || $pk != '_id') {
 			$this->$pk = $value;
 		} else {
@@ -184,7 +184,7 @@ class ActiveRecord extends BaseActiveRecord
 	 */
 	public function getPrimaryKey($asArray = false)
 	{
-		$pk = static::primaryKey();
+		$pk = static::primaryKey()[0];
 		if ($asArray) {
 			return [$pk => $this->$pk];
 		} else {
@@ -197,7 +197,7 @@ class ActiveRecord extends BaseActiveRecord
 	 */
 	public function getOldPrimaryKey($asArray = false)
 	{
-		$pk = static::primaryKey();
+		$pk = static::primaryKey()[0];
 		if ($this->getIsNewRecord()) {
 			$id = null;
 		} elseif ($pk == '_id') {
@@ -213,21 +213,37 @@ class ActiveRecord extends BaseActiveRecord
 	}
 
 	/**
-	 * This method defines the primary.
+	 * This method defines the attribute that uniquely identifies a record.
 	 *
-	 * The primaryKey for elasticsearch documents is always `primaryKey`. It can not be changed.
+	 * The primaryKey for elasticsearch documents is the `_id` field by default. This field is not part of the
+	 * ActiveRecord attributes so you should never add `_id` to the list of [[attributes()|attributes]].
 	 *
-	 * @return string the primary key of this record.
+	 * You may overide this method to define the primary key name when you have defined
+	 * [path mapping](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/mapping-id-field.html)
+	 * for the `_id` field so that it is part of the `_source` and thus part of the [[attributes()|attributes]].
+	 *
+	 * Note that elasticsearch only supports _one_ attribute to be the primary key. However to match the signature
+	 * of the [[\yii\db\ActiveRecordInterface|ActiveRecordInterface]] this methods returns an array instead of a
+	 * single string.
+	 *
+	 * @return string[] array of primary key attributes. Only the first element of the array will be used.
 	 */
 	public static function primaryKey()
 	{
-		return '_id';
+		return ['_id'];
 	}
 
 	/**
 	 * Returns the list of all attribute names of the model.
+	 *
 	 * This method must be overridden by child classes to define available attributes.
-	 * @return array list of attribute names.
+	 *
+	 * Attributes are names of fields of the corresponding elasticsearch document.
+	 * The primaryKey for elasticsearch documents is the `_id` field by default which is not part of the attributes.
+	 * You may define [path mapping](http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/mapping-id-field.html)
+	 * for the `_id` field so that it is part of the `_source` fields and thus becomes part of the attributes.
+	 *
+	 * @return string[] list of attribute names.
 	 */
 	public function attributes()
 	{
@@ -260,7 +276,7 @@ class ActiveRecord extends BaseActiveRecord
 	public static function create($row)
 	{
 		$record = parent::create($row['_source']);
-		$pk = static::primaryKey();
+		$pk = static::primaryKey()[0];
 		$record->$pk = $row['_id'];
 		$record->_score = isset($row['_score']) ? $row['_score'] : null;
 		$record->_version = isset($row['_version']) ? $row['_version'] : null; // TODO version should always be available...
@@ -336,8 +352,11 @@ class ActiveRecord extends BaseActiveRecord
 			if (!isset($response['ok'])) {
 				return false;
 			}
-			$pk = static::primaryKey();
-			$values[$pk] = $this->$pk = $response['_id'];
+			$pk = static::primaryKey()[0];
+			$this->$pk = $response['_id'];
+			if ($pk != '_id') {
+				$values[$pk] = $response['_id'];
+			}
 			$this->_version = $response['_version'];
 			$this->_score = null;
 			$this->setOldAttributes($values);
@@ -362,9 +381,9 @@ class ActiveRecord extends BaseActiveRecord
 	 */
 	public static function updateAll($attributes, $condition = [])
 	{
-		$pkName = static::primaryKey();
+		$pkName = static::primaryKey()[0];
 		if (count($condition) == 1 && isset($condition[$pkName])) {
-			$primaryKeys = (array) $condition[$pkName];
+			$primaryKeys = is_array($condition[$pkName]) ? $condition[$pkName] : [$condition[$pkName]];
 		} else {
 			$primaryKeys = static::find()->where($condition)->column($pkName); // TODO check whether this works with default pk _id
 		}
@@ -372,7 +391,7 @@ class ActiveRecord extends BaseActiveRecord
 			return 0;
 		}
 		$bulk = '';
-		foreach((array) $primaryKeys as $pk) {
+		foreach($primaryKeys as $pk) {
 			$action = Json::encode([
 				"update" => [
 					"_id" => $pk,
@@ -420,9 +439,9 @@ class ActiveRecord extends BaseActiveRecord
 	 */
 	public static function updateAllCounters($counters, $condition = [])
 	{
-		$pkName = static::primaryKey();
+		$pkName = static::primaryKey()[0];
 		if (count($condition) == 1 && isset($condition[$pkName])) {
-			$primaryKeys = (array) $condition[$pkName];
+			$primaryKeys = is_array($condition[$pkName]) ? $condition[$pkName] : [$condition[$pkName]];
 		} else {
 			$primaryKeys = static::find()->where($condition)->column($pkName); // TODO check whether this works with default pk _id
 		}
@@ -430,7 +449,7 @@ class ActiveRecord extends BaseActiveRecord
 			return 0;
 		}
 		$bulk = '';
-		foreach((array) $primaryKeys as $pk) {
+		foreach($primaryKeys as $pk) {
 			$action = Json::encode([
 				"update" => [
 					"_id" => $pk,
@@ -478,9 +497,9 @@ class ActiveRecord extends BaseActiveRecord
 	 */
 	public static function deleteAll($condition = [])
 	{
-		$pkName = static::primaryKey();
+		$pkName = static::primaryKey()[0];
 		if (count($condition) == 1 && isset($condition[$pkName])) {
-			$primaryKeys = (array) $condition[$pkName];
+			$primaryKeys = is_array($condition[$pkName]) ? $condition[$pkName] : [$condition[$pkName]];
 		} else {
 			$primaryKeys = static::find()->where($condition)->column($pkName); // TODO check whether this works with default pk _id
 		}
@@ -488,7 +507,7 @@ class ActiveRecord extends BaseActiveRecord
 			return 0;
 		}
 		$bulk = '';
-		foreach((array) $primaryKeys as $pk) {
+		foreach($primaryKeys as $pk) {
 			$bulk .= Json::encode([
 				"delete" => [
 					"_id" => $pk,
diff --git a/extensions/yii/gii/generators/crud/Generator.php b/extensions/yii/gii/generators/crud/Generator.php
index 7e0e671..bbd2ab5 100644
--- a/extensions/yii/gii/generators/crud/Generator.php
+++ b/extensions/yii/gii/generators/crud/Generator.php
@@ -176,7 +176,7 @@ class Generator extends \yii\gii\Generator
 		/** @var \yii\db\ActiveRecord $class */
 		$class = $this->modelClass;
 		$pk = $class::primaryKey();
-		return is_array($pk) ? $pk[0] : $pk;
+		return $pk[0];
 	}
 
 	/**
@@ -363,7 +363,7 @@ class Generator extends \yii\gii\Generator
 	{
 		/** @var ActiveRecord $class */
 		$class = $this->modelClass;
-		$pks = (array) $class::primaryKey();
+		$pks = $class::primaryKey();
 		if (count($pks) === 1) {
 			return "'id' => \$model->{$pks[0]}";
 		} else {
@@ -379,7 +379,7 @@ class Generator extends \yii\gii\Generator
 	{
 		/** @var ActiveRecord $class */
 		$class = $this->modelClass;
-		$pks = (array) $class::primaryKey();
+		$pks = $class::primaryKey();
 		if (count($pks) === 1) {
 			return '$id';
 		} else {
@@ -391,7 +391,7 @@ class Generator extends \yii\gii\Generator
 	{
 		/** @var ActiveRecord $class */
 		$class = $this->modelClass;
-		$pks = (array) $class::primaryKey();
+		$pks = $class::primaryKey();
 		if (($table = $this->getTableSchema()) === false) {
 			$params = [];
 			foreach ($pks as $pk) {
diff --git a/extensions/yii/gii/generators/crud/templates/controller.php b/extensions/yii/gii/generators/crud/templates/controller.php
index 7430b78..bb5d3f4 100644
--- a/extensions/yii/gii/generators/crud/templates/controller.php
+++ b/extensions/yii/gii/generators/crud/templates/controller.php
@@ -19,7 +19,7 @@ if ($modelClass === $searchModelClass) {
 
 /** @var ActiveRecordInterface $class */
 $class = $generator->modelClass;
-$pks = (array) $class::primaryKey();
+$pks = $class::primaryKey();
 $urlParams = $generator->generateUrlParams();
 $actionParams = $generator->generateActionParams();
 $actionParamComments = $generator->generateActionParamComments();
diff --git a/framework/yii/data/ActiveDataProvider.php b/framework/yii/data/ActiveDataProvider.php
index e1175a8..2292286 100644
--- a/framework/yii/data/ActiveDataProvider.php
+++ b/framework/yii/data/ActiveDataProvider.php
@@ -128,7 +128,7 @@ class ActiveDataProvider extends BaseDataProvider
 		} elseif ($this->query instanceof ActiveQueryInterface) {
 			/** @var \yii\db\ActiveRecord $class */
 			$class = $this->query->modelClass;
-			$pks = (array) $class::primaryKey();
+			$pks = $class::primaryKey();
 			if (count($pks) === 1) {
 				$pk = $pks[0];
 				foreach ($models as $model) {
diff --git a/framework/yii/db/BaseActiveRecord.php b/framework/yii/db/BaseActiveRecord.php
index 133c9b6..4219507 100644
--- a/framework/yii/db/BaseActiveRecord.php
+++ b/framework/yii/db/BaseActiveRecord.php
@@ -1273,7 +1273,7 @@ abstract class BaseActiveRecord extends Model implements ActiveRecordInterface
 	 */
 	public static function isPrimaryKey($keys)
 	{
-		$pks = (array) static::primaryKey();
+		$pks = static::primaryKey();
 		if (count($keys) === count($pks)) {
 			return count(array_intersect($keys, $pks)) === count($pks);
 		} else {
diff --git a/framework/yii/validators/UniqueValidator.php b/framework/yii/validators/UniqueValidator.php
index 55c96cf..51474b0 100644
--- a/framework/yii/validators/UniqueValidator.php
+++ b/framework/yii/validators/UniqueValidator.php
@@ -101,7 +101,7 @@ class UniqueValidator extends Validator
 			$n = count($objects);
 			if ($n === 1) {
 				$keys = array_keys($params);
-				$pks = (array) $targetClass::primaryKey();
+				$pks = $targetClass::primaryKey();
 				sort($keys);
 				sort($pks);
 				if ($keys === $pks) {
diff --git a/tests/unit/data/ar/elasticsearch/Customer.php b/tests/unit/data/ar/elasticsearch/Customer.php
index 1ff1cef..22d2c09 100644
--- a/tests/unit/data/ar/elasticsearch/Customer.php
+++ b/tests/unit/data/ar/elasticsearch/Customer.php
@@ -22,7 +22,7 @@ class Customer extends ActiveRecord
 
 	public static function primaryKey()
 	{
-		return 'id';
+		return ['id'];
 	}
 
 	public function attributes()
diff --git a/tests/unit/data/ar/elasticsearch/Item.php b/tests/unit/data/ar/elasticsearch/Item.php
index 2474f60..033c38d 100644
--- a/tests/unit/data/ar/elasticsearch/Item.php
+++ b/tests/unit/data/ar/elasticsearch/Item.php
@@ -14,7 +14,7 @@ class Item extends ActiveRecord
 {
 	public static function primaryKey()
 	{
-		return 'id';
+		return ['id'];
 	}
 
 	public function attributes()
diff --git a/tests/unit/data/ar/elasticsearch/Order.php b/tests/unit/data/ar/elasticsearch/Order.php
index 41d05f7..e7607cd 100644
--- a/tests/unit/data/ar/elasticsearch/Order.php
+++ b/tests/unit/data/ar/elasticsearch/Order.php
@@ -15,7 +15,7 @@ class Order extends ActiveRecord
 {
 	public static function primaryKey()
 	{
-		return 'id';
+		return ['id'];
 	}
 
 	public function attributes()
diff --git a/tests/unit/extensions/elasticsearch/ActiveRecordTest.php b/tests/unit/extensions/elasticsearch/ActiveRecordTest.php
index 89431aa..2d31792 100644
--- a/tests/unit/extensions/elasticsearch/ActiveRecordTest.php
+++ b/tests/unit/extensions/elasticsearch/ActiveRecordTest.php
@@ -251,7 +251,7 @@ class ActiveRecordTest extends ElasticSearchTestCase
 
 	public function testInsertNoPk()
 	{
-		$this->assertEquals('id', Customer::primaryKey());
+		$this->assertEquals(['id'], Customer::primaryKey());
 		$pkName = 'id';
 
 		$customer = new Customer;