- What do persistent classes look like? How transparent is the persistence tool? Do we have to adopt a programming model and conventions for classes of the business domain?
- How is mapping metadata defined? Because the object/relational transformation is governed entirely by metadata, the format and definition of this metadata is important. Should an ORM tool provide a GUI interface to manipulate the metadata graphically? Or are there better approaches to metadata definition?
- How do object identity and equality relate to database (primary key) identity? How do we map instances of particular classes to particular table rows?
- How should we map class inheritance hierarchies? There are several standard strategies. What about polymorphic associations, abstract classes, and interfaces?
- How does the persistence logic interact at runtime with the objects of the business domain? This is a problem of generic programming, and there are a number of solutions including source generation, runtime reflection, runtime bytecode generation, and build-time bytecode enhancement. The solution to this problem may affect your build process (but, preferably, shouldn’t otherwise affect you as a user).
- What is the lifecycle of a persistent object? Does the lifecycle of some objects depend upon the lifecycle of other associated objects? How do we translate the lifecycle of an object to the lifecycle of a database row?
- What facilities are provided for sorting, searching, and aggregating? The application could do some of these things in memory, but efficient use of relational technology requires that this work often be performed by the database.
- How do we efficiently retrieve data with associations? Efficient access to relational data is usually accomplished via table joins. Object-oriented applications usually access data by navigating an object network. Two data access patterns should be avoided when possible: the n+1 selects problem, and its complement, the Cartesian product problem (fetching too much data in a single select).
(Java persistence with Hibernate.)
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