What's the Difference Between `@JoinColumn` and `@mappedBy` in JPA Relationships?

Unveiling the Distinction between @JoinColumn and @mappedBy in JPA Relationships

In JPA, managing relationships between entities is crucial for data integrity and retrieval. Two annotations, @JoinColumn and @mappedBy, play distinct roles in establishing these relationships.

@JoinColumn: Indicating Ownership and Foreign Key Column

The @JoinColumn annotation marks an entity as the owner of a relationship. This means that the corresponding table will contain a column with a foreign key reference to the referenced table. In the example provided:

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

The Company entity owns the relationship with its Branch entity. The @JoinColumn annotation specifies that a column named "companyIdRef" in the Branch table will hold the foreign key to the "companyId" column in the Company table.

@mappedBy: Indicating Inverse Relationship and Owner in Opposite Entity

On the other hand, the @mappedBy attribute marks an entity as the inverse of a relationship. The attribute name specified in @mappedBy should match the name of the property in the opposing entity that represents the relationship from its perspective.

In the example:

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

The @mappedBy attribute indicates that the Branch entity is the inverse of the relationship with respect to the Company entity. As "companyIdRef" matches the property name in the Branch entity:

@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "companyId")
private Company company;

Consequences of Annotation Usage

• @JoinColumn: The entity annotated with @JoinColumn owns the relationship, meaning it controls the foreign key column.
• @mappedBy: The entity annotated with @mappedBy is the inverse of the relationship, and the owner is declared in the opposing entity. Bi-directional access is available through both entities.

Conclusion

Understanding the difference between @JoinColumn and @mappedBy is essential for establishing correct relationships between JPA entities. By utilizing these annotations appropriately, developers can maintain data integrity and seamlessly navigate complex object graphs.

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