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EclipseLink/DesignDocs/374771
Design Specification: TREAT expression support
Document History
| Date | Author | Version Description & Notes |
|---|---|---|
| 20 March 2012 | Gordon Yorke | Initial Document created |
Project overview
JPA 2.1 will add support for a TREAT function within JPQL and criteria. Support will be required for both JPQL and Criteria queries.
Concepts
When creating a JOIN in a JPQL statement or Criteria API expression the type of the join is that of the corresponding attribute type. Should the query writer wish to reference an attribute of a subclass of that JOIN type the query writer would use the TREAT function to effectively join to a subclass of the JOIN type and JOIN is treated as the specified subtype.
Select e from Employee e, JOIN TREAT(e.projects as LargeProject) l WHERE l.budget > 25000
In the case of this example the type of Employee.projects is Project. Because the query writer wants to access an attribute of the subclass LargeProject TREAT is used.
Requirements
Support use of the TREAT keyword in JPQL and Criteria API for FROM, WHERE and SELECT clauses. Ensure proper validation exceptions occur. Of the 3 inheritance strategies, single table inheritance (STI), joined-table inheritance (JTI), and table per class (TPC), TREAT will only be required to work with the first 2 (STI and JTI). Treat with Table per class inheritance maybe left as a seperate issue
Design Constraints
1) In the case of joins, the referenced object does not participate in the result. Type filtering will need to be added to the query and inner joins to subclasses for JTI. 2) In the case of a restriction, the associated predicate is false. Type filtering will need to be added to the predicate for STI, and use outer joins/parallel expressions for subclasses with JTI.
Design / Functionality
Testing
API
GUI
Config files
Documentation
Work Required
- Implement expression framework support for WHERE clause
- approx 3 days
- Implement JPQL support
- approx 2 day
- Implement Criteria support
- approx 2 days
- Testing
- approx 2 days
Open Issues
This section lists the open issues that are still pending that must be decided prior to fully implementing this project's requirements.
| Issue # | Owner | Description / Notes |
|---|---|---|
| 1 | Deciding if the shouldReadSubclasses inheritance flag should affect this feature. For instance, if false, should treat(person as Employee) only include employees, or should it include Employee subclasses as well. |
Decisions
This section lists decisions made. These are intended to document the resolution of open issues or constraints added to the project that are important.
| Issue # | Description / Notes | Decision |
|---|---|---|
Future Considerations
During the research for this project the following items were identified as out of scope but are captured here as potential future enhancements. If agreed upon during the review process these should be logged in the bug system. 1) support for Treat with table per class inheritance.