Foreign Key Mapping

Maps an association between objects to a foreign key reference between tables.

• Overview
• How It Works
• When to Use It

Overview

Objects can refer to each other directly by object references. To save these objects to a database, we need to keep these referecens.

How It Works

• Each object contains the database key from the appropriate database table (Identity Field).

• If two objects are linked together with an association, this can be replaced by a foreign key in the database.

• If you have a collectio nof objects, you have to reverse the direction of the reference.

• To avoid getting lost in cycles, you have two choices regarding how you create your objects.

  • Creation method includes data that will give you a fully formed object but you place Lazy Load at appropriate points to break the cycles.
  • Create empty objects and immediately put them in an Identity Map, so when you cycle bvack around, the object is already loaded and you’ll end the cycle.

When to Use It

A Foreign Key Mapping can be used for almost all associations between classes.

• The most common case where it isn’t possible is with many-to-many associations. Foreign keys are single values, and first normal form means that you can’t store multiple foreign keys in a single field. Instead, you need to use Association Table Mapping.

• If you have a collection field with no back pointer, you should consider whether the many side should be a Dependent Mapping, which can simplify your handling of the collection.

• If the related obejct is a Value Object thedn you should use Embedded Value.