Attribute references

When you find yourself creating a new attribute that have already been defined on another data structure, use attribute reference to avoid copy&paste.

For example, suppose there are data structures defining account number and account details:

<data-structure name="Account number" code="AccountNumber">
        <attribute name="Prefix" code="prefix" status="optional" type="integer"
                length="6" description="Account number prefix." />
        <attribute name="Number" code="number" status="mandatory" type="integer"
                length="10" description="The account number itself." />
        ...
</data-structure>
<data-structure name="Account details" code="AccountDetails">
        <attribute name="Currency" code="currency" status="mandatory" type="Currency" />
        <attribute name="Name" code="name" status="mandatory" type="string"
                description="Name of the primary account holder." />
        ...
</data-structure>

Now, you want to create a new data structure that will define a query form to find accounts. You want to be able to find accounts by account number prefix and number, and by account currency, name and balance.

Apart from balance, all four remaining attributes are already defined on the previous data structures, so instead of copy&pasting them, we just refer to them using the attribute-ref element:

<data-structure name="Account query form" code="account-query-form">
        <description>A form to find accounts.</description>
        <attribute-ref type="AccountNumber.prefix" />
        <attribute-ref type="AccountNumber.number" status="optional" />
        <attribute-ref type="AccountDetails.currency" status="optional" />
        <attribute-ref type="AccountDetails.name" status="optional" />
        <attribute name="Available balance" type="decimal" />
</data-structure>

The type of attribute-ref contains a reference to an attribute defined on (another) data structure. The attribute-ref defines an attribute which takes over all the properties of the referred attribute. You can also override some properties, as described in table below. Attribute collection property cannot be overridden.

Attributes of attribute-ref element:

This approach is particularly suitable in situation, when you have the entities / domain objects defined (and relatively fixed). You can then construct other data structures, like UI forms, interface transfer objects, etc. by putting together attributes from the entities / domain objects using attribute references.