Working with Transformation Mappings

Creating Transformation Mappings

Use this procedure to create transformation mappings in the OracleAS TopLink Mapping Workbench.

To create a transformation mapping:

1. In the Navigator pane select the attribute to be mapped.

2. 
   Description of the illustration trmapbtn.gif
   
   Click the Transformation Mapping button from the Mapping toolbar.

   Figure 5-10 Transformation Mapping Tab

   
   Description of the illustration trmaptab.gif
   
   

3. Use the Database Row --> Object Method drop-down list to select a method to convert the database row into an object.

   
   

   Note: The method must have the parameter (DatabaseRow) or parameters (DatabaseRow, Session).

   
   

4. Click Add to add field transformation methods to the descriptor.

   To remove a transformation method, select the method and click Remove.

5. Select the Use indirection option to specify if the creation of the target object requires extensive computational resources. If selected, OracleAS TopLink uses indirection objects. See "Working with Indirection" on page 6-5 for more information.

6. After specifying the details of the mapping, create the attribute field transformation methods in the associated Java class (see Example 5–3).

You can also specify:

• Read-only attributes – See "Specifying Read-Only Settings" on page 4-72

• Access methods – See "Specifying Direct Access and Method Access" on page 4-71

// Get method for the normalHours attribute since method access indicated access public Time[] getNormalHours()
{ 
    return normalHours;
}
// Set method for the normalHours attribute since method access indicated access public void setNormalHours(Time[] theNormalHours)
{ 
    normalHours = theNormalHours;
}
// Create attribute transformation method to read from the database row
//** Builds the normalHours Vector. IMPORTANT: This method builds the value but does not set it. The mapping will set it using method or direct access as defined in the descriptor. */
public Time[] getNormalHoursFromRow(DatabaseRow row)
{ 
    Time[] hours = new Time[2];
    hours[0] = (Time)row.get("START_TIME");
    hours[1] = (Time)row.get("END_TIME");
    return hours;
}
// Define a field transformation method to write out the start time. Return the first element of the normalHours attribute.
public java.sql.Time getStartTime()
{ 
    return getNormalHours()[0];
}
// Define a field transformation method to write out the end time. Return the last element of the normalHours attribute.
public java.sql.Time getEndTime()
{ 
    return getNormalHours()[1];
}

Specifying Advanced Features Available by Amending the Descriptor

In OracleAS TopLink, transformation mappings do not require you to specify an attribute.

A field can be mapped from a computed value that does not map to a logical attribute. This, in effect, constitutes a write-only mapping. In the OracleAS TopLink Mapping Workbench, all mappings are associated with an attribute before any other information can be specified. Therefore, to use a write-only mapping, you must build it by amending the descriptor. The mapping itself has no attribute name, get and set methods, or attribute method. In your amendment method, create an instance of TransformationMapping and send addFieldTransformation() message for each field to be written.

public static void addToDescriptor(Descriptor descriptor) {
// Create a Transformation mapping and add it to the descriptor.
TransformationMapping transMapping = new transMapping.addFieldTransformation(ÒWRITE_DATEÓ, Òdescriptor.addMapping(transMapping);
}

public void addToDescriptor(Descriptor descriptor) {
    TransformationMapping keyMapping = new TransformationMapping();
    keyMapping.addFieldTranslation(ÒPROJECT.PROJ_TYPEÓ,
        ÒgetTypeÓ);descriptor.addMapping(keyMapping);}

Define the getType method on the class to return its type value:

Project>>public abstract String getType();
LargeProject>>public String getType() { return ÒLÓ; }
SmallProject>>public String getType() { return ÒSÓ; }

Refer to "Amending Descriptors After Loading" on page 4-27 for more information.