ORM Relationships

There are 2 prevalent persistence specifications in the Java ORM world. JDO2 provides the most complete definition, whilst JPA is the most recent. In this guide we show the different types of ORM relation commonly used, and mark against it which specification supports it. This list is not yet complete but will be added to to provide a comprehensive list of relationship type and where you can find it.

Field Type	Relation	JDO2	JPA2	DataNucleus
PC	1-1 Unidirectional
PC	1-1 Bidirectional
PC	1-1 serialised
PC	1-1 CompoundIdentity Unidirectional
PC	1-N CompoundIdentity Bidirectional
Interface	1-1 Unidirectional
Interface	1-1 Bidirectional
Interface	1-1 serialised		?
Collection<PC>	1-N ForeignKey Unidirectional Collection
Collection<PC>	1-N ForeignKey Bidirectional Collection
Collection<PC>	1-N JoinTable Unidirectional Collection
Collection<PC>	1-N JoinTable Bidirectional Collection
Collection<Non-PC>	1-N JoinTable Collection
Collection<PC>	1-N JoinTable Collection using shared JoinTable
Collection<PC>	1-N ForeignKey Collection using shared ForeignKey
Collection<PC>	M-N JoinTable
Collection<PC>	1-N CompoundIdentity Unidirectional
Collection<PC>	1-N serialised Collection
Collection<PC>	1-N JoinTable Collection of serialised elements
List<PC>	1-N ForeignKey Unidirectional Indexed List
List<PC>	1-N ForeignKey Bidirectional Indexed List
List<PC>	1-N JoinTable Unidirectional Indexed List
List<PC>	1-N JoinTable Bidirectional Indexed List
List<Non-PC>	1-N JoinTable Indexed List
List<PC>	1-N ForeignKey Unidirectional Ordered List
List<PC>	1-N ForeignKey Bidirectional Ordered List
List<PC>	1-N JoinTable Unidirectional Ordered List
List<PC>	1-N JoinTable Bidirectional Ordered List
Map<PC, PC>	1-N JoinTable Map
Map<Non-PC, PC>	1-N JoinTable Map
Map<PC, Non-PC>	1-N JoinTable Map
Map<Non-PC, Non-PC>	1-N JoinTable Map
Map<Non-PC, PC>	1-N ForeignKey Map Unidirectional (key stored in value)
Map<Non-PC, PC>	1-N ForeignKey Map Bidirectional (key stored in value)
Map<PC, Non-PC>	1-N ForeignKey Map Unidirectional (value stored in key)
Map<PC, PC>	1-N serialised Map
Map<PC, PC>	1-N JoinTable Map of serialised keys/values
PC[ ]	1-N ForeignKey Unidirectional Array
PC[ ]	1-N JoinTable Unidirectional Array
PC[ ]	1-N serialised Array
Non-PC[ ]	1-N JoinTable Unidirectional Array