_key ] = array(
			'title' => sprintf( __( 'Child %s', 'jet-engine' ), $label_2 ),
			'type'  => 'select',
			'name'  => $meta_key,
		);

		$this->_relations_for_post_types[ $post_type_2 ][ $meta_key ] = array(
			'title' => sprintf( __( 'Parent %s', 'jet-engine' ), $label_1 ),
			'type'  => 'select',
			'name'  => $meta_key,
		);

	}

	/**
	 * Get values for relations meta fields early
	 *
	 * @param  [type] $result  [description]
	 * @param  [type] $post    [description]
	 * @param  [type] $key     [description]
	 * @param  [type] $default [description]
	 * @param  [type] $field   [description]
	 * @return [type]          [description]
	 */
	public function get_meta( $result, $post, $key, $default, $field ) {

		$relation = isset( $this->_active_relations[ $key ] ) ? $this->_active_relations[ $key ] : false;

		if ( ! $relation ) {
			return $result;
		}

		$meta = get_post_meta( $post->ID, $key, false );
		$meta = array_filter( $meta );
		$meta = array_values( $meta );

		if ( ! isset( $field['multiple'] ) || ! $field['multiple'] ) {
			$meta = isset( $meta[0] ) ? $meta[0] : null;
		}

		if ( ! empty( $meta ) ) {
			return $meta;
		} else {
			return null;
		}

	}

	/**
	 * Check if is relation meta key
	 *
	 * @param  [type]  $meta_key [description]
	 * @return boolean           [description]
	 */
	public function is_relation_key( $meta_key = null ) {
		return isset( $this->_active_relations[ $meta_key ] );
	}

	/**
	 * Returns active relations list
	 *
	 * @return [type] [description]
	 */
	public function get_active_relations( $rel_id = false ) {
		return $this->_active_relations;
	}

	/**
	 * Synchronize related meta on post save
	 *
	 * @return void
	 */
	public function process_meta( $result = null, $post_id = null, $meta_key = '', $related_posts = array() ) {

		$relation = isset( $this->_active_relations[ $meta_key ] ) ? $this->_active_relations[ $meta_key ] : false;

		if ( ! $relation ) {
			return $result;
		}

		if ( empty( $related_posts ) ) {
			$related_posts = ! empty( $_POST[ $meta_key ] ) ? $_POST[ $meta_key ] : false;
		}

		if ( empty( $related_posts ) ) {
			jet_engine()->relations->data->delete_all_related_meta( $meta_key, $post_id );
		}

		if ( ! is_array( $related_posts ) ) {
			$related_posts = array( $related_posts );
		}

		$prev_related    = get_post_meta( $post_id, $meta_key );
		$saved_post_type = get_post_type( $post_id );
		$type            = explode( '_to_', $relation['type'] );

		if ( $relation['post_type_1'] === $saved_post_type ) {
			$current_type = $type[0];
			$related_type = $type[1];
		} elseif ( $relation['post_type_2'] === $saved_post_type ) {
			$current_type = $type[1];
			$related_type = $type[0];
		} else {
			return $result;
		}

		$result    = true;
		$to_delete = array_diff( $prev_related, $related_posts );
		$to_delete = array_filter( $to_delete );

		if ( ! empty( $to_delete ) ) {
			foreach ( $to_delete as $delete_post_id ) {
				delete_post_meta( $delete_post_id, $meta_key, $post_id );
				delete_post_meta( $post_id, $meta_key, $delete_post_id );
			}
		}

		foreach ( $related_posts as $related_post_id ) {

			if ( $related_post_id && ! in_array( $related_post_id, $prev_related ) ) {
				add_post_meta( $post_id, $meta_key, $related_post_id, false );
			}

			$stored = get_post_meta( $related_post_id, $meta_key, false );

			if ( ! is_array( $stored ) ) {
				$stored = array( $stored );
				$stored = array_filter( $stored );
			}

			if ( 'one' === $current_type ) {
				foreach ( $stored as $stored_id ) {
					if ( absint( $stored_id ) !== $post_id ) {
						delete_post_meta( $related_post_id, $meta_key );
						delete_post_meta( $stored_id, $meta_key, $related_post_id );
					}
				}
			}

			if ( $post_id && ! in_array( $post_id, $stored ) ) {
				add_post_meta( $related_post_id, $meta_key, $post_id, false );
			}

		}

		return $result;

	}

	/**
	 * Returns relation meta keys for passed post type
	 *
	 * @param  string $post_type Post type name
	 * @return array
	 */
	public function get_relation_fields_for_post_type( $post_type = null ) {

		$default = array();

		if ( ! $post_type ) {
			return $default;
		}

		if ( empty( $this->_relations_for_post_types ) ) {
			return $default;
		}

		if ( empty( $this->_relations_for_post_types[ $post_type ] ) ) {
			return $default;
		}

		return $this->_relations_for_post_types[ $post_type ];

	}

	/**
	 * Returns info about relationby hash
	 */
	public function get_relation_info( $key ) {
		return isset( $this->_active_relations[ $key ] ) ? $this->_active_relations[ $key ] : false;
	}

	/**
	 * Returns related posts
	 *
	 * @return void
	 */
	public function get_related_posts( $args = array() ) {

		$post_type_1 = isset( $args['post_type_1'] ) ? $args['post_type_1'] : false;
		$post_type_2 = isset( $args['post_type_2'] ) ? $args['post_type_2'] : false;
		$hash        = isset( $args['hash'] ) ? $args['hash'] : false;

		if ( $hash ) {
			$meta_key = $hash;
			$relation = isset( $this->_active_relations[ $meta_key ] ) ? $this->_active_relations[ $meta_key ] : false;

			if ( ! $relation ) {
				return false;
			}

			$post_type_1 = $relation['post_type_1'];
			$post_type_2 = $relation['post_type_2'];

			$current = ! empty( $args['current'] ) ? $args['current'] : false;

			if ( $current ) {
				$args['from'] = ( $post_type_1 === $current ) ? $post_type_2 : $post_type_1;
			}

		} else {

			if ( ! $post_type_1 || ! $post_type_2 ) {
				return false;
			}

			$meta_key = $this->get_relation_hash( $post_type_1, $post_type_2 );
			$relation = isset( $this->_active_relations[ $meta_key ] ) ? $this->_active_relations[ $meta_key ] : false;
		}

		if ( ! $relation ) {
			return false;
		}

		$post_id = isset( $args['post_id'] ) ? $args['post_id'] : get_the_ID();
		$type    = explode( '_to_', $relation['type'] );
		$from    = isset( $args['from'] ) ? $args['from'] : $post_type_1;

		if ( $post_type_1 === $from ) {
			$type = $type[0];
		} else {
			$type = $type[1];
		}

		if ( 'one' === $type ) {
			$single = true;
		} else {
			$single = false;
		}

		$meta = get_post_meta( $post_id, $meta_key, $single );
		$meta = apply_filters( 'jet-engine/relations/get_related_posts', $meta );

		return $meta;

	}

	/**
	 * Clear relations meta on delete a relation post.
	 *
	 * @param $post_id
	 */
	public function clear_relations_meta_on_delete_post( $post_id ) {

		$post_type       = get_post_type( $post_id );
		$relation_fields = $this->get_relation_fields_for_post_type( $post_type );

		if ( ! empty( $relation_fields ) ) {
			foreach ( $relation_fields as $field => $args ) {
				jet_engine()->relations->data->delete_all_related_meta( $field, $post_id );
			}
		}
	}

}