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 cannot belong to more than one instance of the collection.
  • An instance of the contained entity class cannot appear at more than one value of the collection index.
An association from Product to Part requires the 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.
        class="ClassName"                                   1
        not-found="ignore|exception"                        2
        entity-name="EntityName"                            3


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.
The <one-to-many> element does not need to declare any columns. Nor is it necessary to specify the table name anywhere.


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 for more information.
The following 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"
    <key column="productId" not-null="true"/>
    <map-key formula="partName"/>
    <one-to-many class="Part"/>