JPA Persist parent and child with one to many relationship

Question

I want to persist parent entity with 20 child entities, my code is below

Parent Class

@OneToMany(mappedBy = "parentId") private Collection<Child> childCollection; 

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Child Class

@JoinColumn(name = "parent_id", referencedColumnName = "parent_id") @ManyToOne(optional=false) private Parent parent; 

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String jsonString = "json string containing parent properties and child  collection"   ObjectMapper mapper = new ObjectMapper(); Parent parent = mapper.readValue(jsonString, Parent.class);  public void save(Parent parent) {     Collection<Child> childCollection = new ArrayList<>() ;      for(Child tha : parent.getChildCollection()) {          tha.setParent(parent);         childCollection.add(tha);     }      parent.setChildCollection(childCollection);     getEntityManager().persist(parent);  } 

So if there are 20 child tables then I have to set parent reference in each of them for that I have to write 20 for loops? Is it feasible? is there any other way or configuration where I can automatically persist parent and child?

Answer 1

Fix your Parent class:

@OneToMany(mappedBy = "parent") 

mappedBy property should point to field on other side of relationship. As JavaDoc says:

The field that owns the relationship. Required unless the relationship is unidirectional.

Also you should explicitely persist Child entity in cycle:

for(Child tha : parent.getChildCollection()) {      ...     getEntityManager().persist(tha);     ... } 

As Alan Hay noticed in comment, you can use cascade facilities and let EntityManager automatically persist all your Child entities:

@OneToMany(mappedBy = "parent", cascade = CascadeType.PERSIST) 

More details about cascades (and JPA itself) you can find in Vlad Mihalcea's blog.

Answer 2

Generally, @JoinColumn indicates that the entity is the owner of the relationship & mappedBy indicates that the entity is the inverse of the relationship.

So, if you are trying like following

@OneToMany(mappedBy = "parent") private Collection<Child> childCollection; 

That means it is inverse of the relationship and it will not set parent reference to its child.

To set parent reference to its child, you have to make the above entity owner of the relationship in the following way.

@OneToMany(cascade = CascadeType.ALL) @JoinColumn private Collection<Child> childCollection; 

You need not set any child reference because above code will create a column in the child table.