Why do I have to sync a bidirectional relationship even with CascadeType.PERSIST using Spring Data JPA

Here is an example code demonstrating the issue:

The Meal Entity:

@Entity
public class Meal {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private long id;

  @OneToMany(mappedBy = "meal", cascade = CascadeType.PERSIST)
  private Collection<Food> foods;

  public Meal() {
    foods = new HashSet<>();
  }

  public Collection<Food> getFoods() {
    return foods;
  }

  public void addFood(Food food) {
    foods.add(food);
    // without this the `meal_id` column is null
    food.setMeal(this);
  }
}

The Food Entity:

@Entity
public class Food {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private long id;

  @ManyToOne
  private Meal meal;

  public long getId() {
    return id;
  }

  public void setId(long id) {
    this.id = id;
  }

  public Meal getMeal() {
    return meal;
  }

  public void setMeal(Meal meal) {
    this.meal = meal;
  }
}

And here is the code that creates and saves the entities:

Meal meal = new Meal();

for (int i = 0; i < 10; i++) {
  meal.addFood(new Food());
}

mealRepository.save(meal);

Both the Meal and the Food entities are persisted thanks to the CascadeType.PERSIST, but the meal_id column stays null if I don't explicitly set the meal field to the Meal entity.

This is not the behavior I'd expect, and I'm wondering why isn't this automatically done for me.

@Entity
public class Meal {


  @OneToMany(mappedBy = "meal", cascade = CascadeType.PERSIST)
  private Collection<Food> foods;

} 

The mappedBy here means that the value of meal field in Food is used to provide the value for the corresponding database column that link between Food and Meal (i.e. meal_id column in Food table). So if you do not set it, it will always remain NULL.

On the other hand, without mappedBy means that the value of the foods field in Meal is used to provide the value for meal_id column. The reason of manually syncing them is just to have a consistent OOP model to work with.

CascadeType.PERSIST is nothing to do with setting the value of the entity. It will never update the value of the entities for you. What it helps is to automatically calling some entityManager methods on the related entities. Without it, you have to call the following to save all Food and Meal:

entityManager.persist(meal);
entityManager.persist(food1);
entityManager.persist(food2);
......
entityManager.persist(foodN);

With it , you just need to call

entityManager.persist(meal);

and the rest of entityManager.persist(foodN) will be 'cascaded' to call automatically.

So in term of spring-data-jpa , instead of calling

mealRepository.save(meal);
foodRepository.persist(food1);
foodRepository.persist(food2);
......
foodRepository.persist(foodN);

You just need to call

mealRepository.save(meal);