private function prepare_meta_query_for_relationships() {

		$meta_query = array();

		$relations = array(
			'related_posts' => '_llms_related',
			'engagements'   => '_llms_engagement',
		);
		foreach ( $relations as $arg => $meta_key ) {

			// Include.
			if ( ! empty( $this->get( $arg ) ) ) {
				$meta_query[] = array(
					'key'     => $meta_key,
					'value'   => $this->sanitize_id_array( $this->get( $arg ) ),
					'compare' => 'IN',
				);
			}

			// Exclude.
			$exclude_arg = $arg . '__exclude';
			if ( ! empty( $this->get( $exclude_arg ) ) ) {

				$meta_query[] = array(
					'relation' => 'OR',
					array(
						'key'     => $meta_key,
						'value'   => $this->sanitize_id_array( $this->get( $exclude_arg ) ),
						'compare' => 'NOT IN',
					),
					// Ensure posts that don't have the metadata set will be returned.
					array(
						'key'     => $meta_key,
						'compare' => 'NOT EXISTS',
					),
				);
			}
		}

		return $meta_query;

	}