Working with Nested Table Mappings

Nested table types model an unordered set of elements. These elements may be built-in or user-defined types. You can view a nested table as a single-column table or, if the nested table is an object type, as a muticolumn table (with a column for each attribute of the object type).

Typically, nested tables represent a one-to-many or many-to-many relationship of references to another independent structure. They support querying and joining better than Varrays that are inlined to the parent table.

OracleAS TopLink supports nested table through the NestedTableMapping class. They represent a collection of object references in Java, similar to a OneToManyMapping or ManyToManyMapping. However, the nested table mapping does not require foreign key information (as with a one-to-many mapping) or the relational table (as with a many-to-many mapping).

Implementing Nested Table Mappings in Java

Nested table mappings are instances of the NestedTableMapping class. This mapping is associated to an attribute in the parent class. The following elements are required for a nested table mapping to be viable:

• The attribute being mapped, which is set by sending the setAttributeName() message

• The field being mapped, which is set by sending the setFieldName() message

• The name of the array structure, which is set by sending the setStructureName() message

Use the optional setGetMethodName() and setSetMethodName() messages to allow OracleAS TopLink to access the attribute through user-defined methods, rather than directly. See "Specifying Direct Access and Method Access" on page 4-71 for more information.

Table 7–5 summarizes all nested table mapping properties.

Example 7-5 Nested Table

The following code example illustrates creating a nested table mapping for the Insurance source class and registering it with the descriptor.

// Create a new mapping and register it with the source descriptor.
NestedTableMapping policiesMapping = new NestedTableMapping();
policiesMapping.setAttributeName("policies");
policiesMapping.setGetMethodName("getPolicies");
policiesMapping.setSetMethodName("setPolicies");
policiesMapping.setReferenceClass(Policy.class);
policiesMapping.dontUseIndirection();
policiesMapping.setStructureName("POLICIES_TYPE");
policiesMapping.setFieldName("POLICIES");
policiesMapping.privateOwnedRelationship();
policiesMapping.setSelectionSQLString("select p.* from policyHolders ph, table(ph.policies) t, policies p where ph.ssn=#SSN and ref(p) = value(t)");
descriptor.addMapping(policiesMapping);

Reference

The following table summarizes all nested table mapping properties. In the Method Names column, arguments are bold, methods are not.

Table 7-5 Properties for NestedTableMapping Methods

Property	Default	Method Names
* Required property
Attribute to be mapped *	not applicable	setAttributeName(String name)
Set parent class *	not applicable	setReferenceClass(Class referenceClass)
User-defined data type *	not applicable	setStructureName(String structureName)
Field to be mapped *	not applicable	setFieldName(String fieldName)
Method access	direct access	setGetMethodName(String name) setSetMethodName(String name)
Indirection	use indirection	useIndirection() dontUseIndirection() setUsesIndirection(boolean usesIndirection)
Privately owned relationship	independent	independentRelationship() privateOwnedRelationship() setIsPrivateOwned(Boolean isPrivateOwned)
Read only	read / write	readWrite() readOnly() setIsReadOnly(boolean readOnly)