Hibernate 3 Formulas
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If the formula expression evaluation result is 0--i.e., no sub-product is allowed--then the object will be of the class Product. If the result is 1, the object will be a NestedProduct. In tables 1 and 2, for the first record (ProductID=10000001) in the Product table, the initialized class will be a NestedProduct because it references a ProductRelease record with SubProductAllowable=1. For the second record (ProductID=20000001) in the Product table, the initialized class will be Product, because it references a ProductRelease record with SubProductAllowable=0.

S/N	ProductReleaseID	SubProductAllowable	...
1	11	1	¡­
2	601	0	¡­

Table 1. Records in the ProductRelease Table

S/N	ProductID	ProductReleaseID	...
1	10000001	11	¡­
2	20000001	601	...

Table 2. Records in the Product Table

Property

A formula inside of a property element allows object attributes to contain certain derived values, like the results of sum, average, max, etc., as in:

<property name="averagePrice" formula="(select avg(pc.price) from PriceCatalogue pc, SelectedItems si where si.priceRefID=pc.priceID)"/>

Furthermore, a formula can also help retrieve values from another table based on certain attribute values for the current record. For example:

<property name="currencyName" formula="(select cur.name from currency cur where cur.id= currencyID)"/>

This helps retrieve a currency name from the currency table. As you can see, these direct mappings can eliminate a lot of conversion coding.

map-key

formula allows map-key to have any possible value. In the following example (Figure 3), we want Role_roleID to be the map-key of the object model (Figure 4).

Figure 3. User role data schema

Figure 4. User role object model

In the data schema above, User and Role are linked via a many-to-many relationship table called User_has_Role. In order to get a User with all of the roles assigned to it, we use the following mapping:

<hibernate-mapping>
  <class name="User">
    <id name="userID"/>
    <map name="roles"  
        table="UserRole"/>
      <key column="User_userID"/>
      <map-key 
        formula="Role_RoleID" 
        type="string"/>
      <many-to-many 
        column="Role_RoleID"
        class="Role"/>
    </map>
  </class>
  <class name="Role">
    <id name="roleID"/>
  </class>
</hibernate-mapping>

Role_RoleID is used as the joining column value of the many-to-many element. However, Hibernate does not allow Role_RoleID to be in the column attributes of both map-key and many-to-many. But with a formula, Role_RoleID can be used for map-key as well.

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