Swift 2.1核心数据 - 保存具有一对多关系但不添加异构数据（如果已存在）的数据

I have a ViewController that creates a new recipe data. To create a new recipe, user will need to fill out title, ingredients etc along with selecting its category. And for that I have established Category and Recipe entities with One to Many relationship.I have a ViewController that creates a new reci