Foundations of Object Relational Mapping

Peter Kitson

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Introduction

Object-relational mapping is the process of transforming between object and relational modeling approaches and between the systems that support these approaches. Doing a good job at object-relational mapping requires a solid understanding of object modeling and relational modeling, how they are similar, and how they are different. Ideally, we should have a single integrated model that described both approaches. This would ensure we understand and explicitly document both concepts and their relationships. This document will present what we believe to be the only correct integration of the two worlds that is suitable for implementation on a relational database.

Difficulties occur when we have to deal with the real systems implementing object and relational models. These systems have implementations that are deficient or inconsistent with the theoretical approaches. Relational databases have been deficient for multiple decades in implementing the core concepts of relational theory. On the other hand, object modeling is not standardized, so each programming environment implements its own variation. Because of these deficiencies, object-relational mapping is more complicated than it needs to be.

Fortunately, object modeling and relational modeling have such different concerns that they are actually extremely compatible. Reltaional theory is concerned with and object techniques are primarily concerned with behaviour. Mapping between the two models requires deciding how the two worlds can refer to each other. We will first describe the two worlds in more detail and then show how they can be integrated.