private function isRelationship($model, $conditions)
    {
        /**
         * A valid relationship must:
         * - Be a method
         * - Not appear in the 'except' list
         * - Appear in the 'only' list (if used)
         * - Extend Illuminate\Database\Eloquent\Relations\Relation
         */
        return (
            method_exists($this, $model) &&
            $this->filterRelationshipByConditions($model, $conditions) &&
            is_subclass_of($this->{Str::camel($model)}(), 'Illuminate\Database\Eloquent\Relations\Relation')
        );
    }