protected function normalize_mapping() {
		$mapping = [];

		foreach ( $this->maps as $attribute => $metadata ) {
			// Allowed using same name of attribute and metadata.
			// Eg: [gallery] same as [gallery => gallery].
			$attribute = is_int( $attribute ) ? $metadata : $attribute;

			if ( array_key_exists( $attribute, $this->attributes ) ) {
				$mapping[ $attribute ] = $metadata;
			}
		}

		return $mapping;
	}