6.2.5. One-to-many associations

A one to many association links the tables of two classes via a foreign key, with no intervening collection table. This mapping loses certain semantics of normal Java collections:
  • An instance of the contained entity class may not belong to more than one instance of the collection
  • An instance of the contained entity class may not appear at more than one value of the collection index
An association from Product to Part requires existence of a foreign key column and possibly an index column to the Part table. A <one-to-many> tag indicates that this is a one to many association.
<one-to-many 
        class="ClassName"
        not-found="ignore|exception"
        entity-name="EntityName"
        node="element-name"
        embed-xml="true|false"
    />
class (required): The name of the associated class.
not-found (optional - defaults to exception): Specifies how cached identifiers that reference missing rows will be handled: ignore will treat a missing row as a null association.
entity-name (optional): The entity name of the associated class, as an alternative to class.
Notice that the <one-to-many> element does not need to declare any columns. Nor is it necessary to specify the table name anywhere.
Very important note: If the foreign key column of a <one-to-many> association is declared NOT NULL, you must declare the <key> mapping not-null="true" or use a bidirectional association with the collection mapping marked inverse="true". See the discussion of bidirectional associations later in this chapter.
This example shows a map of Part entities by name (where partName is a persistent property of Part). Notice the use of a formula-based index.
<map name="parts"
        cascade="all">
    <key column="productId" not-null="true"/>
    <map-key formula="partName"/>
    <one-to-many class="Part"/>
</map>