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Professional EJB : The EJB 2.0 Entity Model

Relationship Descriptions

Relationships are described in the deployment descriptor. A relationship represents one meaningful association between any pair of entity beans, based on the business logic of the application. Here are some examples of relationships:

• Between customers and orders
• Between orders and line items
• Between teachers and students
• Between classes and classrooms
• Between automobile manufacturers and automobile makes
• Between automobile makes and product safety recalls

Each relationship consists of two roles. A role is essentially a traversal of the relationship in a particular direction. In other words, a role begins at one endpoint of the pair of relationship participants (known as the role source) and looks to the other participant in the relationship (known as the role target, which the container infers by using the reverse role's source). For each role in a relationship, there are potentially five pieces of information that can be specified (plus the usual assortment of names and descriptions):

• The multiplicity, which can be either One or Many. This multiplicity refers to the quantity of the source object in the relationship.
• The role source. This is the name of the EJB 2.0 CMP entity.
• The <cmr-field> name. This is only necessary if there are corresponding get and set accessors in the source object. For a unidirectional relationship, only one role will have a <cmr-field> name. For a bi-directional relationship, both roles will have a <cmr-field> name. (For any role that has a <cmr-field> name, the source entity of the reverse role must be represented by a local interface.)
• The type of collection for a <cmr-field> that potentially represents more than one target element. The legal values are java.util.Set or java.util.Collection.
• Whether or not the target object should, when deleted, apply a cascade-delete to the source objects participating in this relationship. The target object (that is, the reverse role) must have a multiplicity of one.

In the example of the Order-LineItem relationship, there would be two roles. They might be aptly named OrderHasLineItems, and LineItemInOrder. Let's take a look at the information that we would provide in the deployment descriptor for these roles:

	OrderHasLineItems	LineItemInOrder
multiplicity	One. There is at most one order associated with any line item.	Many. There are potentially many line items associated with a particular order.
role source	The entity OrderEJB.	The entity LineItemEJB.
<cmr-field>	lineItems	In this example, this is a unidirectional relationship between orders and line items, so no <cmr-field> is specified.
type of collection	java.util.Collection	As we did not specify a <cmr-field>, we do not need to specify a type of collection. Even if we had specified a <cmr-field>, we would not need to specify a type of collection because the reverse relationship has a multiplicity of one.
cascade-delete	It would not be legal to specify cascade-delete here, because the reverse relationship (LineItemInOrder) has a multiplicity of many.	The cascade-delete element could be specified here if we wanted to remove all of an order's line items when that order was removed.

Here is a possible deployment descriptor entry for this relationship:

<ejb-relation>
	<ejb-relation-name>OrderLineItem</ejb-relation-name>

	<ejb-relationship-role>
		<ejb-relationship-role-name>
			OrderHasLineItems
		</ejb-relationship-role-name>
		<multiplicity>One</multiplicity>
		<relationship-role-source>
			<ejb-name>Order</ejb-name>
		</relationship-role-source>
		<cmr-field>
			<cmr-field-name>lineItems</cmr-field-name>
			<cmr-field-type>java.util.Collection</cmr-field-type>
		</cmr-field>
	</ejb-relationship-role>

	<ejb-relationship-role>
		<ejb-relationship-role-name>
			LineItemInOrder
		</ejb-relationship-role-name>
		<multiplicity>Many</multiplicity>
		<relationship-role-source>
			<ejb-name>LineItemEJB</ejb-name>
		</relationship-role-source>
	</ejb-relationship-role>

</ejb-relation>

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