What s the difference between JoinColumn and mappedBy when using a JPA OneToMany association

Question

What is the difference between:

@Entity
public class Company {

    @OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY)
    @JoinColumn(name = "companyIdRef", referencedColumnName = "companyId")
    private List<Branch> branches;
    ...
}

and

@Entity
public class Company {

    @OneToMany(cascade = CascadeType.ALL , fetch = FetchType.LAZY, mappedBy = "companyIdRef")
    private List<Branch> branches;
    ...
}

Answer 1

The annotation @JoinColumn indicates that this entity is the owner of the relationship (that is: the corresponding table has a column with a foreign key to the referenced table), whereas the attribute mappedBy indicates that the entity in this side is the inverse of the relationship, and the owner resides in the "other" entity. This also means that you can access the other table from the class which you've annotated with "mappedBy" (fully bidirectional relationship).

In particular, for the code in the question the correct annotations would look like this:

@Entity
public class Company {
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "company")
    private List<Branch> branches;
}

@Entity
public class Branch {
    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "companyId")
    private Company company;
}

Comments

Hello All,

I could not understand the part "This also means that you can access the other table from the class which you've annotated with "mappedBy" (fully bidirectional relationship)". Till now, I do understand that the relationship made by using the mapped by and join column is unidirectional <in one to one association>, "mappedBy" only signifies the owning entity.

How you are saying the relationship is bidirectional and not unidirectional? could you please explain?

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Hi, @There,

Note that: The main difference is that the bidirectional relationship provides navigational access in both directions so that you can access the other side without explicit queries. Also it allows you to apply cascading options to both directions.

Answer 2

JPA mapping annotation can be classified as object model annotation and relational model annotation.

Object model annotation is used to specify object data model while relational model annotation to relational data model. Separating these two type annotation into different file to decouple object model and relational data model is the beast practice

@OneToMany is object model annotation and declares a one-to-many unidirectional relationship between entity object. The declared relationship detail can modified with attributes of the annotation @OneToMany.

@JoinColum is relational model annotation. It assumes that the object relationship is mapped as join column rather than table , and declares that  in table corresponding to the owner side entity(entity at many-side in one-to-many object relationship) there is a join column references table corresponding to inverse side(entity at one-side in one-to-many object relationship)