How do I map a many-to-one association in Hibernate where the child has a composite key and a part of that is the primary key in the parent?

Question

I have a (legacy) table structure that looks a little bit like this:

table parent (
  parentid int (PK)
  ...
)

table child (
  parentid int (PK)
  childid int (PK)
  ...
)

That is, part of the primary key of child is the primary key for parent. I have tried to map this (we use xml-based Hibernate) like this:

<hibernate-mapping>
<class name="com.example.MyParent" table="parent">

    <id name="parentid" column="parentid" unsaved-value="0" >
        <generator class="identity"/>
    </id>

    <set name="children" cascade="all">
        <key>
            <column name="parentid"/>
        </key>
        <one-to-many class="com.example.MyChild" />
    </set>
...
</class>

<class name="com.example.MyChild" table="child">
    <composite-id name="id" class="com.example.MyChildId">
        <key-property name="parentid" column="parentid" />
        <key-property name="childid" column="childid" />
    </composite-id>
    ...
</class>

Java class:

public void updateParent(MyParent param) {
    ht.saveOrUpdate(param);

}

UPDATE: I had used the wrong relation type (updated), but now I have another problem: It seems that when creating the child rows in the table, the parentid is null. Since parentid is part of the composite key, they insert fails.

From the log:

DEBUG SQL - insert into Child(PARENTID, CHILDID) values (?, ?)
TRACE IntegerType - binding null to parameter: 1
TRACE IntegerType - binding '5678' to parameter: 2
WARN  JDBCExceptionReporter - SQL Error: -10, SQLState: 23502
ERROR JDBCExceptionReporter - integrity constraint violation: NOT NULL check constraint;     SYS_CT_10028 table: Child

Answer 1

I think you have two issues here:

1. Many-to-one should be declared in the MyChild class (if I understand correctly).
2. When using @JoinColumn annotation with composite keys, referencedColumnName is mandatory. Maybe this applies to XML as well...? See http://en.wikibooks.org/wiki/Java_Persistence/Identity_and_Sequencing#Primary_Keys_through_OneToOne_and_ManyToOne_Relationships

PS: if childid is already identifying (unique, not-null), there's no need to have the parentid in the key (FK would be enough).

Answer 2

What's in the ... in the child mapping? Does it by any chance declare a property called parent which is mapped to the parentid column?

Answer 3

正如汤姆·安德森（Tom Anderson）所说，您将需要在子代与父代之间进行多对一映射，并且可能在父集映射上使用inverse = true，以使休眠状态知道子代正在管理该关系。