Github project to reproduce: https://raw.githubusercontent.com/tmonney/hibernate-mapsid

Suppose a pair of entities defined with the same composite primary key:

@Entity
public class Parent

{code|title=Child.java}
@Entity
public class Child {
    @EmbeddedId
    private Key id;

    @OneToOne
    @MapsId
    @JoinColumns({
            @JoinColumn(name = "id1", referencedColumnName = "id1"),
            @JoinColumn(name = "id2", referencedColumnName = "id2")
    })
    public Parent parent;

    @Basic
    public String value;
}

@Embeddable
public class Key implements Serializable {

private static final long serialVersionUID = 1L;

@Column(name = "id1")
public Long id1;

@Column(name = "id2")
public Long id2;

public Key() {
}

public Key(Long id1, Long id2)

@Override
public int hashCode()

@Override
public boolean equals(Object obj) {
if (!(obj instanceof Key))

Key that = (Key) obj;
return Objects.equals(this.id1, that.id1) && Objects.equals(this.id2, that.id2);
}
}

The bug manifests itself when loading a {{Parent}} entity, getting its {{child}}, modifying some of its fields and finally flushing the entity manager:

Batch update returned unexpected row count from update [0]; actual row count: 0; expected: 1

This is because Hibernates executes the following query:
{code:sql}
update FailingChild set value=? where id1=? and id2=?

And passes the same value for id1 and id2, which is obviously wrong.

Interestingly enough, everything works OK if the mapping of the {Child class is changed as follows (notice the second join column of the parent is renamed):

@Entity
public class Child {
@EmbeddedId
public Key id;

@OneToOne
@MapsId
@JoinColumns(

)
public Parent parent;

@Basic
public String value;
}