Allow for the composition of relations defined in roles.

[jonathanstowe]

The #80 and #84 provide for the composition of columns defined in roles, however it would equally useful to have relations defined in roles too.

So that something like:

model State { ... }
model Workflow is table('tinky_workflow') is rw {
    has Int     $.id                is serial;
    has Str     $.name              is column(:unique);
    has State   @.states            is relationship({ .workflow-id });
}

role WithWorkflow {
    has Int         $.workflow-id   is referencing( { Workflow.id });
    has Workflow    $.workflow      is relationship({ .workflow-id });
}

model State is table('tinky_state') does WithWorkflow is rw {
    has Int $.id            is serial;
    has Str $.name          is column;
}

Would work.

As it currently stands the add-relationship being called in the trait code itself is happening too early as the role meta object doesn't have that method. I think the easiest way to do this is to add a new parametric role to the attribute in the traits and then have a compose-relationships that would be called in the model class compose which can enumerate those attributes and then call the add-relationship at that point.

[FCO]

A rakudo bug make it impossible to use those closures ({ Workflow.id } and { .workflow-id }). I have a patch that should make relationships on roles work, but I can’t test it because of this bug. https://github.com/rakudo/rakudo/issues/2619