Multiple many-to-many associations in one Grails domain class

Question

I am using Grails 3.0.6 and am struggling with a complicated and highly interconnected domain model. I have classes with multiple many-to-many associations to other classes and I am left with no choice but to have multiple belongsTo associations on at least one class. I am unable to figure out the syntax to represent this.

My domain model was quite complicated, but I was able to reduce my problem to this simplified example:

class Graph {
    static hasMany = [vertices: Vertex]
}

class OtherClass {
    static hasMany = [vertices: Vertex]
}

class Vertex {
    static hasMany = [graph: Graph, other: OtherClass]
}

In this simplified example, I could get around the problem by declaring the ownership between the domain classes on Graph and OtherClass... In my complicated domain model, I don't have this choice because there are too many classes with multiple many-to-many associations.

I have tried this:

class Vertex {
    static hasMany = [graphs: Graph, others: OtherClass]
    static belongsTo = Graph, OtherClass
}

but I get an NPE.

I have tried this:

class Vertex {
    static hasMany = [graphs: Graph, others: OtherClass]
    static belongsTo = [graphs: Graph, others: OtherClass]
}

but I still get "GrailsDomainException: No owner defined between domain classes [Graph] and [Vertex]"

Is there something I could do with mappedBy to correctly represent this?

In many of my many-to-many associations, cascading saves are not actually wanted (although they won't hurt), so I don't need belongsTo (or an "owner") for that purpose. This makes me wonder if associations on the domain classes are really how I should be modeling these relationships. Is there something else I could be doing?

Answer 1

per Burt Beckwith's comment, I created an additional domain class to represent the join table. Now, one many-to-many association is broken down into two one-to-many associations and the problem does not arise.

Example:

class Graph {
    static hasMany = [graphVertexRelations: GraphVertexRelation]
}

class OtherClass {
    static hasMany = [vertices: Vertex]
}

class Vertex {
    static hasMany = [graphVertexRelations: GraphVertexRelation, others: OtherClass]
    static belongsTo = OtherClass
}

class GraphVertexRelation {
    static belongsTo = [graph: Graph, vertex: Vertex]

    static GraphVertexRelation create(Graph graph, Vertex vertex, boolean flush = false) {
        new GraphVertexRelation(graph: graph, vertex: vertex).save(flush: flush, insert: true)
    }
}