Hibernate creates two primary keys when I only want one

I have a Hibernate class called Expression (开发者_如何学Csimplified here for your viewing pleasure):

@Entity
public class Expression {
    @Id
    @GeneratedValue
    private long id;

    private String data;

    @OneToMany(fetch=FetchType.EAGER)
    @Cascade({CascadeType.MERGE, CascadeType.PERSIST})
    private Set<Expression> dependencies;
}

This creates two tables, Expression(id, data) and Expression_Expression(expression_id, dependencies_id). However, Hibernate sets both the expression_id and dependencies_id columns as the primary key so I can't have a duplicate dependencies_id, which is what I'd like.

For example, if I have three Expressions:

Expression x = new Expression("0");
Expression y = new Expression("1");
Expression z = new Expression("x + y");
Set<Expression> tmp = new HashSet<Expression>();
tmp.add(x);
tmp.add(y);
z.setDependencies(tmp);
// Persist x, y, and z.

then Hibernate will perform the following:

Insert x into the Expression table with id = 1 and data = "0"

Insert y into the Expression table with id = 2 and data = "1"

Insert z into the Expression table with id = 3 and data = "a + b"

Insert a row into the 'Expression_Expression' table with expression_id = 3 and dependencies_id = 1. (Making z a dependent of x.)

But when it tries to insert a row into the Expression_Expression table with expression_id = 3 and dependencies_id = 2 (making z into a dependent of y) I get a duplicate entry error.

I'd like to be able to have multiple rows in the Expression_Expression table with the same expression_id value.

Any assistance would be greatly appreciated!

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You should use the @ManyToMany relation here