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For the very same reason, the strategy also limits the ability to enforce data integrity constraints. For example, if you want to enforce a column
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constraint that BID_FREQUENCY cannot be NULL for a Bidder, you would not be able to enforce the constraint since the same column will contain Seller records for which the value may be NULL. Typically, such constraints are enforced through alternative mechanisms such as database triggers for the single-table strategy.
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The second inheritance strategy avoids the pitfalls we ve described and fully utilizes database relationships.
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8.4.2 Joined-tables strategy
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The joined-tables inheritance strategy uses one-to-one relationships to model OO inheritance. In effect, the joined-tables strategy involves creating separate tables for each entity in the OO hierarchy and relating direct descendants in the hierarchy with one-to-one relationships. For a better grasp, let s see how the data in figure 8.10 might look using this strategy. In the joined-tables strategy, the parent of the hierarchy contains only columns common to its children. In our example, the USERS table contains columns common to all ActionBazaar user types (such as USERNAME). The child table in the hierarchy contains columns specific to the entity subtype. Here, both the BIDDERS and SELLERS tables contain columns specific to the Bidder and Seller entities, respectively (for example, the SELLERS table contains the CREDIT_WORTH column). The parent-child OO hierarchy chain is implemented using one-to-one relationships. For example, the USERS and SELLERS tables are related through the USER_ID foreign key in the SELLERS table pointing to the primary key of the USERS table. A similar relationship exists between the BIDDERS and USERS tables. The discriminator column in the USERS table is still used, primarily as a way of easily differentiating
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Figure 8.10 Modeling inheritance using joined tables. Each entity in the OO hierarchy corresponds to a separate table and parent-child relationships are modeled using one-to-one mapping.
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data types in the hierarchy. Listing 8.13 shows how the mapping strategy is implemented in EJB 3.
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Listing 8.13 Inheritance mapping using joined tables
@Entity @Table(name="USERS") @Inheritance(strategy=InheritanceType.JOINED) @DiscriminatorColumn(name="USER_TYPE", discriminatorType=STRING, length=1) public abstract class User ... @Entity @Table(name="SELLERS") @DiscriminatorValue(value="S") @PrimaryKeyJoinColumn(name="USER_ID") public class Seller extends User ... @Entity @Table(name="BIDDERS") @DiscriminatorValue(value="B") @PrimaryKeyJoinColumn(name="USER_ID") public class Seller extends User ...
Primary key join
Primary key join
Listing 8.13 uses the @DiscriminatorColumn and @DiscriminatorValue annotations in exactly the same way as the single-table strategy. The @Inheritance annotation s strategy element is specified as JOINED, however. In addition, the one-toone relationships between parent and child tables are implemented through the @PrimaryKeyJoinColumn annotations in both the Seller and Bidder entities. In both cases, the name element specifies the USER_ID foreign key. The joined-tables strategy is probably the best mapping choice from a design perspective. From a performance perspective, it is worse than the single-table strategy because it requires the joining of multiple tables for polymorphic queries.
8.4.3 Table-per-class strategy
Table-per-class is probably the simplest inheritance strategy for a layperson to understand. However, this inheritance strategy is the worst from both a relational and OO standpoint. In this strategy, both the superclass (concrete class) and subclasses are stored in their own table and no relationship exists between any of the tables. To see how this works, take a look at the tables in figure 8.11. As figure 8.11 shows, entity data are stored in their own tables even if they are inherited from the superclass. This is true even for the USER_ID primary key. As a
Figure 8.11 The table-per-class inheritance strategy. Super- and subclasses are stored in their own, entirely unrelated tables.
result, primary keys in all tables must be mutually exclusive for this scheme to work. In addition, inherited columns are duplicated across tables, such as the USERNAME column. Using this inheritance strategy, we define the strategy in the superclass and map tables for all the classes. Listing 8.14 shows how the code might look.