Declaring Entity Relationships

There are two kinds of relationships:

OneToOne - is used to represent 1:1 relationships between entities.
ManyToOne - is used to represent N:1 relationships between entities.

The following relationship DOM tags are recognized and optional:

[affects.primary.key] : [true or false] indicates that the primary key of the first end of the relationship is affected by this relationship (default false).
[java.isReferenceInverted] : [true or false] indicates whether the relationship should be inverted or not in the generated java code. By default, relationships are inverted.

When a relation is affecting the primary key, the relation is implicitly required (not nullable).

A relation that is not affecting the primary key is by default optional (nullable). To declare that a relation is not nullable, you need to use the required keyword as follows:

Precedence { second required } to Activity { predecessors }

For example, the following sample code declares a OneToOne relationship between two entities:

entity Resource {
  // DOM [primary.keys] : [id]
  id String required,
  name String
}

entity ResourceCapacity {
  quantity Integer
}

relationship OneToOne {
  // DOM [affects.primary.key] : [true]
  ResourceCapacity{resource} to Resource{capacity}
}

This declaration affects the entities as follows:

the Resource entity has three attributes (an id, a name of type String and a capacity of type ResourceCapacity).
the primary key of a Resource entity is built from id. The ResourceCapacity entity has two attributes (quantity of type Integer and resource of type Resource).
the primary key of a ResourceCapacity entity is built from the primary key of the attached Resource.

The following example declares a ManyToOne relationship between two entities:

entity Activity {
     // DOM [primary.keys] : [id]
     id String required,
     name String,
     durationInHours Integer
}
   
entity Precedence {
}

relationship ManyToOne {
  // DOM [affects.primary.key] : [true]
  Precedence{first required} to Activity{successors},
  // DOM [affects.primary.key] : [true]
  Precedence{second required} to Activity{predecessors}
}

This declaration affects the entities as follows:

the Activity entity has five attributes (id and name of type String, durationInHours of type Integer, while successors and predecessors are arrays of type Precedence)
the Precedence entity has two attributes (first and second of type Activity) that cannot be null
the primary key of a Precedence entity is built from the primary keys of the attached first and second activities.

When using the generated java code, setting an activity in the first attribute of a precedence will automatically add the precedence in the successors list of that activity. Similarly, removing a precedence from the successors list of an activity will automatically set the first attribute of that precedence to null.