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Association Classes

You normally don't create an entity class for a join table.

  • The film_actor table, for example, is made of keys which are essential for the @ManyToMany relationship, but building an entity for this table would not add any additional value to our codebase.

However, sometimes the join table contains additional data that does not belong with either entity that participates in the many-to-many relationship, but instead belongs to the association itself.

  • The inventory_item table, for example, contains a column called media_condition that wouldn't make sense to place in Film or Store.

  • If this is the case, you can create an entity for the join table to model the additional data.

  • This new class is sometimes called an association class.

Drill

  1. Create an enum called MediaCondition with values of New, Used, Damaged, Lost, and NA.

  2. Create a new entity called InventoryItem which is mapped to the inventory_item table. Add fields for

    • id
    • mediaCondition
    +-----------------+------------------------------------------+------+-----+-------------------+-----------------------------+
| Field           | Type                                     | Null | Key | Default           | Extra                       |
+-----------------+------------------------------------------+------+-----+-------------------+-----------------------------+
| id              | int(10) unsigned                         | NO   | PRI | NULL              | auto_increment              |
| film_id         | int(10) unsigned                         | NO   | MUL | NULL              |                             |
| store_id        | smallint(5) unsigned                     | NO   | MUL | NULL              |                             |
| media_condition | enum('New','Used','Damaged','Lost','NA') | YES  |     | NULL              |                             |
| last_update     | timestamp                                | NO   |     | CURRENT_TIMESTAMP | on update CURRENT_TIMESTAMP |
+-----------------+------------------------------------------+------+-----+-------------------+-----------------------------+

  3. Build a JUnit test for the InventoryItem entity to ensure it is mapped correctly.

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