Logical datamodels and disjoint subtyping - eclipse-efbt/efbt GitHub Wiki

Logical datamodels and disjoint subtyping

BIRD contains a logical datamodel and a relational model. The logical model contains a concept of ‘inheritance’ or ‘subtyping’ which is very easy to describe as inheritance Python Django Models (given that Python as a language allows inheritence as it is an Object Oriented language). It also contains a concept of ‘disjoint subtyping’ to describe concepts that group together entities from multiple hierarchies (known as arcs in SQLDeveloper). We describe this same concept of disjoint subtyping in Django as a ‘delegate’, this is a pattern sometimes known as ‘preferring aggregation over inheritance’, we show a typical comparison here.

In SQL developer we show here that Instrument table has 2 ‘arcs’ which contain sublclasses. The right hand arc is ‘is instrument type by origin ‘ containing subtypes ‘Instrument resulting directly from a Financial contract’ and ‘Instrument resulting directly from a Credit Facility’ and the right hand is ‘instrument type by product’ containing ‘Over the counter (OTC) Derivative instrument’ , ‘Off-balance instrument’ , ‘Securities financing transaction (SFT)’ and 2 others not in this screenshot. In a simplified manner we can say that we are going to need to choose one subtype from each arc when we represent an instrument. We show the 2 arcs here in blue, and then show refined diagrams showing the content of each arc:

Contents of ‘Instrument by Product’ Arc:

Contents of ‘Instrument by Origin’ Arc:

In UML, and therefore in Python Django we describe the same as

Note that the arrows black diamonds refer to ‘composition relationship’ in UML, and the lines with white arrow heads are inheritance/subtyping. This allows the same case as in disjoint subtyping where we must choose a subtype from each delegate.