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3. Basic Collection (one-to-many)


Definition:
A one-to-many reference is basically a collection. Here a first-rank class, A, holds a reference to a collection of another first-rank class, B.

Scenario:
We have two first-rank classes, Foo and Bar which are related to each other as follows:

Set Foo.getBars() // returns a collection of Bar instances

Hibernate Mapping:
In Hibernate, this could be mapped as follows:

<class name="Foo" table="foo">
     ...
     <set role="bars" table="bar">
         <key column="foo_id"/>
         <one-to-many class="Bar"/>
     </set>
 </class>

NB: We will mostly use <set> type collections in these examples except where we are examining specific collection-type features. Please refer to the hibernate documentation to learn about the comprehensive set of collection types available.

Table Schema:

Foo

id

Bar

id

foo_id


Now we have created an extra column in Bar's table which holds the FK to Foo. This allows Foo to be assigned a collection of Bars based on the value of the foo_id column in Bar.

Bidirectionality:
This relationship can be declared both ways, with Bar having getFoo(), by suitable code changes to Bar and the following schema change:

<class name="Bar" table="bar">
     ...
     <many-to-one name="foo" class="Foo" column="foo_id"/>
 </class>

Now your Bars will know who their Foo is. NB: No extra columns are generated for the bidirectionality.

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