April 2003
Beginner to intermediate
380 pages
12h 25m
English
Content preview from Java Data Objects
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Modeling Constructs in Java and Relational Models
The Java object model and the relational data model are two separate and distinct data models with separate type systems and approaches for representing data and expression computations. Table 5-1 summarizes the typical data-specific mappings that are specified between an object model and a relational schema.
Table 5-1. Mapping between object models and relational schemas
Java modeling construct | Relational modeling construct |
|---|---|
Class | Table |
Field | Column |
Instance | Row |
Identity | Primary key |
Reference | Foreign key |
Interface | No relational equivalent |
Collection | No relational equivalent |
Class inheritance | One or multiple tables |
Collections in JDO can be represented only as memory instances, with no direct representation as a collection in the datastore. They are instantiated on demand and discarded when they are no longer needed. There are exceptions to these general rules, and some implementations support more advanced mappings. This chapter examines several ways of representing a Java collection in a relational datastore.
If you start with a set of Java classes and let the JDO implementation generate a relational schema for them, it will choose an appropriate relational representation of your Java model and define the mapping between your classes and the relational tables. The implementation will make a number of relational schema design decisions, including choosing names for tables and columns, column types for your Java fields, and how collections and relationships ...
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