Object/Relational Mapping

Identity Manager uses (Java) objects to perform its work, but when these objects are to be exported to a set of relational database tables, the objects must undergo a transformation commonly called object/relational mapping. This transformation is necessary because there are differences between the types of data that can be expressed in a RDBMS relationship and the types of data that can be expressed in an arbitrary Java object. For example, consider the following Java class:

class Widget {
  private String _id;
  private Map<String,Widget> _subWidgets;
  ...
}

This class presents a problem when expressed in relational terms, because the _subWidgets field is a nested structure. If you try decomposing two hierarchies of Widget objects that have shared subWidgets into a set of RDBMS tables, and delete one of the hierarchies, you quickly end up with a reference-counting problem.

To address the representational differences, Identity Manager places some constraints on what type of data can be exported. Specifically, the limit allows for the top-level Java object to contain scalar attributes, lists of scalar attributes, and maps of scalar attributes. In a few instances, Identity Manager needs a slightly richer expression, and to resolve these cases Identity Manager has introduced the PseudoModel. A PseudoModel is conceptually a data structure containing only scalar attributes. A top-level Java object can contain attributes that are PseudoModels or Lists of PseudoModels. PseudoModels are Identity Manager structures that cannot be extended. The following is an example of a PseudoModel.

class TopLevelModel
{
    private String _name;
    private List<PseudoModelPoint> _points;
}
class PseudoModelPoint
{
    private String _name;
    private String _color;
    private int _x;
    private int _y;
    private int _z;
}

Identity Manager can properly perform the object/relational transformation of TopLevelModel because PseudoModelPoint only contains scalar attributes. In query-notation, the color attribute of the PseudoModel is addressable as:

TopLevelModel.points[].color

When inspecting the Identity Manager Data Export schema, you will find a few PseudoModel types. These types represent some of the more complex data in the top-level export models. You cannot query for a PseudoModel directly because a PseudoModel is not exported directly. A PseudoModel is simply structured data held by an attribute of a top-level model.