What is the significance of the TABLE_PER_CLASS inheritance strategy?

Table of Contents

• Introduction
  • The Role of the TABLE_PER_CLASS Inheritance Strategy
  • Key Features of TABLE_PER_CLASS Strategy:
  • Advantages of TABLE_PER_CLASS Strategy:
  • Disadvantages of TABLE_PER_CLASS Strategy:
• How to Implement the TABLE_PER_CLASS Strategy in JPA
  • Example:
    • 1. Define the Superclass Entity
    • 2. Define the Subclass Entities
  • 3. Resulting Database Schema
    • Vehicle Table:
    • Car Table:
    • Bike Table:
  • 4. Query Example
• Conclusion

Introduction

In Java Persistence API (JPA), inheritance strategies define how entities in a class hierarchy are mapped to database tables. The TABLE_PER_CLASS inheritance strategy is one of the strategies used for this mapping. Under this strategy, each class in the inheritance hierarchy (including the superclass and all its subclasses) is mapped to its own separate table. The main distinction of the TABLE_PER_CLASS strategy is that it avoids joins between superclass and subclass tables, unlike the JOINED strategy, but it can lead to data duplication.

The Role of the TABLE_PER_CLASS Inheritance Strategy

The TABLE_PER_CLASS strategy creates a separate table for each class in the inheritance hierarchy, including the superclass and each subclass. These tables are completely independent of each other, meaning that there is no shared table for the superclass. Each subclass has its own table with all the attributes defined in that subclass (including the inherited attributes). This approach effectively avoids JOIN operations and provides a more direct mapping of each entity to a table.

Key Features of TABLE_PER_CLASS Strategy:

1. Separate Tables for Each Class: Unlike other strategies, each class in the hierarchy gets its own table, regardless of whether it’s a superclass or subclass.
2. No Foreign Key Relationships: There are no foreign key relationships between the tables, as each class’s table is independent.
3. No JOIN Operations: Since each class is stored in a separate table, queries do not require joins between tables for data retrieval, which can be beneficial for performance.
4. Data Duplication: The superclass attributes are duplicated across each subclass table. This can lead to some level of data redundancy.

Advantages of TABLE_PER_CLASS Strategy:

1. Simpler Queries: Since there are no joins involved in queries, the database can be queried more simply and potentially faster for certain types of operations, especially when only one subclass is queried.
2. Independence of Tables: Each subclass table is independent of the others. This makes it easier to manage and scale subclasses that have distinct attributes, and each table is optimized for the specific entity’s data.
3. No JOIN Overhead: The TABLE_PER_CLASS strategy avoids the overhead of JOIN operations, which can improve the performance of simple queries that only access a single subclass.
4. Flexibility for Schema Evolution: As each class has its own table, schema changes, such as adding or removing columns, affect only the relevant table. This makes the schema more flexible for changes to subclasses.

Disadvantages of TABLE_PER_CLASS Strategy:

1. Data Duplication: Since each subclass includes the attributes of the superclass in its table, there is data duplication. The superclass fields are repeated across each subclass table, leading to storage inefficiencies and potential data inconsistency.
2. Harder to Query Across Hierarchy: If you need to query across the entire class hierarchy, you may need to manually manage the union of results from each subclass table, which can be more complex and less efficient than using the JOINED strategy.
3. Lack of Referential Integrity: There is no foreign key relationship between subclass tables and the superclass table, which means the database doesn’t enforce referential integrity between them.

How to Implement the TABLE_PER_CLASS Strategy in JPA

To implement the TABLE_PER_CLASS inheritance strategy in JPA, you use the @Inheritance annotation with strategy = InheritanceType.TABLE_PER_CLASS. This strategy requires each class in the hierarchy to have its own table, and JPA will map the entire inheritance hierarchy using separate tables for each class.

Here’s an example of how to configure the TABLE_PER_CLASS strategy in JPA:

Example:

1. Define the Superclass Entity

The superclass contains the common attributes shared by all subclasses. It is annotated with @Inheritance and strategy = InheritanceType.TABLE_PER_CLASS.

2. Define the Subclass Entities

Each subclass is annotated with @Entity and has its own specific attributes, but no foreign key relationships are established between the tables.

In this example:

• Vehicle is the superclass containing common fields like id and model.
• Car and Bike are subclasses, each having their own specific fields (doors for Car and hasPedals for Bike).
• The @Inheritance(strategy = InheritanceType.TABLE_PER_CLASS) annotation specifies that the TABLE_PER_CLASS inheritance strategy should be used.

3. Resulting Database Schema

Under the TABLE_PER_CLASS strategy, JPA will generate separate tables for Vehicle, Car, and Bike. These tables will look like this:

Vehicle Table:

id	model
1	Sedan
2	MTB

Car Table:

id	model	doors
1	Sedan	4

Bike Table:

id	model	hasPedals
2	MTB	true

In this schema:

• Each table is independent, meaning that each class’s data is stored separately.
• The Car table has its own id, model, and doors columns.
• The Bike table has its own id, model, and hasPedals columns.

Since each subclass has its own table, no foreign key relationships are needed. The fields of the superclass (model in this case) are duplicated in both the Car and Bike tables.

4. Query Example

To retrieve data, you can directly query any subclass without worrying about joins. For example, to query for all Car entities:

Similarly, to query for Bike entities:

However, if you want to query across the entire hierarchy, you may need to manually handle the union of results from each subclass table:

This query will require multiple tables to be queried separately and combined (which JPA will handle as long as you query the superclass), but it can be more complex than using other inheritance strategies like JOINED.

Conclusion

The TABLE_PER_CLASS inheritance strategy in JPA is a mapping strategy where each class in the inheritance hierarchy is stored in its own table. This approach can provide simpler queries and avoids the overhead of JOIN operations. It is useful when each subclass has a distinct set of fields and you want to avoid relationships between tables.

However, the TABLE_PER_CLASS strategy comes with some trade-offs:

• It can lead to data duplication as the superclass attributes are repeated in every subclass table.
• There is no referential integrity between superclass and subclass tables, which can make data management more challenging.
• While it avoids JOINs, querying across the entire class hierarchy can become complex.

Overall, this strategy is ideal for cases where subclasses are highly independent, and you want to avoid the overhead of joins or complex relationships. It is a good choice when schema flexibility and the independence of tables are important considerations.