APIGateway Model Schema does not work with Join

Hi

I have a model schema, and want to use Join within the schema to pull in the correct absolute reference to another model schema.

(Note - to simplify, I have actually just used Join here to join some strings to make up the URL).

What seems to happen is that Troposphere converts the JSON into a string, but doesn't like the Join function.

`test_schema = t.add_resource(Model(
"test",
Name = "test",
ContentType="application/json",
RestApiId=Ref(testApi),
Schema={
"$schema": "http://json-schema.org/draft-04/schema#",
"title": "test",
"id": "/test",
"type": "object",
"properties": {
"_id": {"type": "string"},
"trades": {
"type": "array",
"items": [{"$ref": Join("",["https://apigateway.amazonaws.com/restapis/", "abc123456", "/models/testreference"] ) } ]
}
}
}
))`

I've also tried using the "Fn::Join" native Cloudformation JSON, and that does allow the JSON file to be built without error, but escapes the Fn::Join into a string, so that when CloudFormation runs, it just treats that as the reference without doing the join.