Constraint between relationships (OR operator)

[fmione]

Feature description

It woulde be nice to add a constraint definition for cases where a node could have one or the other relationship (similar to the OR operator), but not both.

For example, consider the following (dummy) case: a Person can only have one type of pet, either a Dog or a Cat.

class Dog(StructuredNode):
    name= StringProperty(required=True)

class Cat(StructuredNode):
    name= StringProperty(required=True)

class Person(StructuredNode):
    name= StringProperty(required=True)

    cat= RelationshipTo('Cat', 'HAS_PET', cardinality=ZeroOrMore)
    dog= RelationshipTo('Dog', 'HAS_PET', cardinality=ZeroOrMore)

This is, of course, a simplified example to illustrate the required constraint, without the need for creating an Animal superclass.

[mariusconjeaud]

First off : dogs and cats CAN live together, why would you want to separate them ?? I heard the tip is the dog should be there before the cat, otherwise the cat will see the dog as an invader.

Serious considerations set aside, here is a proposition of syntax :

class JealousDog(AsyncStructuredNode):
    name = StringProperty(required=True)


class JealousCat(AsyncStructuredNode):
    name = StringProperty(required=True)


class ExclusivePerson(AsyncStructuredNode):
    name = StringProperty(required=True)

    # Define mutually exclusive relationships
    cat = AsyncRelationshipTo(
        "JealousCat",
        "HAS_PET",
        cardinality=AsyncMutuallyExclusive,
        exclusion_group=["dog"],
    )
    dog = AsyncRelationshipTo(
        "JealousDog",
        "HAS_PET",
        cardinality=AsyncMutuallyExclusive,
        exclusion_group=["cat"],
    )

[mariusconjeaud]

(thinking on a way to extend this so you can have a real cardinality + exclusion_group ; this was more a proof of concept than anything else)