Can a @ManyToOne JPA relation be null?

问题

I have a table that has foreign key of another table (many to one relationship) but i want it to be nullable.

Something like this:

public class SubType() {

    @Id
    @GeneratedValue(generator = "system-uuid")
    @GenericGenerator(name = "system-uuid", strategy = "uuid")
    private String id;

}

public class TopUp {

    @Column(nullable = true)
    @ManyToOne(optional = false, fetch = FetchType.LAZY)
    private SubType subType;

}

But @Column(nullable = true) throws the NullPointerException and says subtype can not be null. Is there any way to get the ManyToOne accept null?

回答1:

You need to set:

@ManyToOne(optional = true, fetch = FetchType.LAZY)

not optional=false.

The @Column(nullable=true) is to instruct the DDL generation tool to include a NULL SQL column type constraint.

For more on optional vs nullable, check out this StackOverflow answer.

回答2:

try this:

@JoinColumn(name = "subType_id", nullable = true)

回答3:

Only this helped me:

public class TopUp {
    @ManyToOne
    @JoinColumn(columnDefinition="integer", name="subtype_id")
    private SubType subType;
}

columnDefinition is a solution!

来源：https://stackoverflow.com/questions/25718229/can-a-manytoone-jpa-relation-be-null