Incorrect Hibernate Mapping of OneToOne Relationships with Composite Keys in a PostgreSQL Database

Question

I'm encountering a problem in a Spring Boot application using Hibernate, where OneToOne relationships involving entities with composite keys are not mapped correctly in a PostgreSQL database. Specifically, Hibernate maps the same entity instance to two properties in a parent entity, despite different expected results based on the underlying database entries.

Here are the simplified entity structures to illustrate the problem:

Entities:

@Entity
public class Foo {
    @EmbeddedId
    private FooId id;

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumns({
        @JoinColumn(name = "alphaId", referencedColumnName = "userId"),
        @JoinColumn(name = "betaId", referencedColumnName = "barId")
    })
    private Bar alphaBar;

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumns({
        @JoinColumn(name = "betaId", referencedColumnName = "userId"),
        @JoinColumn(name = "alphaId", referencedColumnName = "barId")
    })
    private Bar betaBar;
}

@Entity
public class Bar {
    @EmbeddedId
    private BarId id;
}

@Embeddable
public class FooId {
    private String alphaId;
    private String betaId;
}

@Embeddable
public class BarId {
    private String userId;
    private String barId;
}

Problem:

In the application, each Foo instance involves two distinct Bar instances, representing different relationships (alphaBar and betaBar). However, when retrieving Foo, both alphaBar and betaBar are populated with the same Bar instance, even though the underlying data in the PostgreSQL database for each Bar is distinct and correct.

Requirements:

Each Foo should map to two distinct Bar instances, reflecting the unique userId and barId combinations. Correct mapping is crucial for the integrity of the application's data handling and processing.

What I've Tried:

Verified database integrity to ensure distinct and accurate data for Bar entries. Checked the generated SQL through Hibernate's logging, which seems correct but does not align with the application's entity mapping results.

Questions:

Is there an error in how I've configured the @JoinColumns for mapping the relationships using composite keys? Could Hibernate's caching or session management be affecting how these entities are uniquely identified and fetched in a PostgreSQL setting? I would appreciate any insights or suggestions on how to resolve this issue.

Update:

Further investigation into the issue has revealed that the problem might be linked to the use of the IN clause in the SQL generated by Hibernate. Despite configuring @JoinColumns for my OneToOne relationships and expecting Hibernate to use an inner join to fetch each associated Bar instance distinctly, the framework still utilizes an IN clause which seems to be causing incorrect entity mapping.

I am seeking advice on how to force Hibernate to utilize an explicit inner join without resorting to an IN clause, or any configurations that might prevent this behavior. Insights into how Hibernate handles such complex entity relationships and any potential misconfigurations in my setup would be highly appreciated.

Answer 1

The problem is in the two @JoinColumns in PostgreSQL, alphaId and betaId, that Hibernate is told to re-use for saving two different Java references, resulting in the collision.

@Entity
public class Foo {
    @EmbeddedId
    private FooId id;

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumns({
        @JoinColumn( name = "alphaId"/*__________________    */
                    ,referencedColumnName = "userId"),/* \   */
        @JoinColumn( name = "betaId"/*_________________   |  */         
                    ,referencedColumnName = "barId")/* \  |  */
    })/*                                                | |  */ 
    private Bar alphaBar;/*                             | |  */ 
    /*                                                  | |  */ 
    @OneToOne(fetch = FetchType.LAZY)/*                 | |  */ 
    @JoinColumns({/*                                    | |  */ 
        @JoinColumn( name = "betaId"/*already used_____/  |  */
                    ,referencedColumnName = "userId"),/*  |  */
        @JoinColumn( name = "alphaId"/*already used______/   */
                    ,referencedColumnName = "barId")
    })
    private Bar betaBar;
}

From the db's perspective, there's no difference between alphaBar and betaBar, because they're encoded by the same two columns in Foo's table on the db.

It's all fine from the Java perspective: these are all valid references to different objects. The decorators instructing Hibernate how to map them to a relational structure are to blame for confusing the alpha and beta Bars.

Assuming that you want each Foo to be related to two (possibly different) Bars, you need to let the alphaBar use its own two columns and give another two to be used for betaBar:

@Entity
public class Foo {
    @EmbeddedId
    private FooId id;

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumns({
        @JoinColumn(name = "alphaBarUserId", referencedColumnName = "userId"),
        @JoinColumn(name = "alphaBarBarId", referencedColumnName = "barId")
    })
    private Bar alphaBar;

    @OneToOne(fetch = FetchType.LAZY)
    @JoinColumns({
        @JoinColumn(name = "betaBarUserId", referencedColumnName = "userId"),
        @JoinColumn(name = "betaBarBarId", referencedColumnName = "barId")
    })
    private Bar betaBar;
}

Depending on your PhysicalNamingStrategy that dictates how Hibernate maps your field names in Java to column names in PostgreSQL, it's possible Foo's composite primary key is also trying to use those same two columns, because FooId is also constructed with an alphaId and betaId fields. If Hibernate translates that literally to "alphaId" and "betaId", it would mean Foo gets just one pair of columns and tries to save all three things (its own composite identifier as well as the two references) in the same one pair of columns.