To have a class perform a DB mapping for its subclasses (a generic idea)
- Some objects will certainly appear in the context of another object.
- At this point, a mapping of the first object is performed using the mapper of another object to simplify the mapping process.
- Operating mechanism
- When DB is persisted, the dependency class relies on the owner class. Each dependent can have only one owner.
- Activity record and row data entry
- The mapping code for the dependency class is written in the owner.
- Data mapper
- A mapper class that does not have a dependency, which completes the dependency mapping code in the owner's mapper.
- Table Data Entry
- There is no dependency class at all. Complete the processing of the dependents in the owner.
- Typically, when an owner is loaded, the dependent dependents are loaded. Lazy loading is used when the associated load is expensive.
- Dependent person
- No domain is identified. It does not have to be stored in an identity map.
- cannot be loaded by the lookup method by ID . Thus there is no finder for the relying person, but only the owner's finder.
- One dependency may also be the owner of another dependency. At this point, the master owner controls the entire dependency hierarchy.
- In a Memory object, only the owner and the dependent of the relying person can have a reference to the relying person.
- In DB, other tables cannot have foreign keys to the dependency table, except for the owner table and the table that owns the owner.
- In UML, a combination is used to represent the relationship between the two.
- Update
- By the owner to complete the write and save of the dependent person.
- Updates to the dependents can be done by deleting and inserting. The process does not need to be analyzed (directly removed first, then inserted).
- However, tracking changes to the owner can become complex.
- All changes to the dependent person are marked by the owner.
- Scenario: Keep the dependent person constant.
- In this way, the change to the dependent person is removed first and then inserted into a new dependent person process.
- This scenario simplifies the DB mapping, but simultaneously coupled the memory model and database mappings.
- Use time
- When an object is preparing another object reference, it is generally when an object has a corresponding set of dependents, while the relying person does not have a back pointer.
- at the same time, it simplifies the retention management of objects if the object itself does not require an ID.
- prerequisites to be met
- only one owner per dependent
- cannot exist outside the owner of an object that has a reference to the relying person.
- It is a technique that simplifies db mapping and makes it impossible to reference dependents externally. The discovery mechanism that causes the root owner is complex.
- do not use a unit of work when working with it. The
- unit of work cannot control the dependents. The result is an orphaned, non-tracked dependency.
