schema.Reference: Optional Query parameter

[smyrman]

It would be useful if we could extend the schema.Reference with an optional field Query query.Predicate that would allow us to specify restrictions on which items could legally be linked.

type Reference struct {
	Path string
	Query query.Predicate
}

Example (somewhere in Schema.Fields):

	"ingredient": {
		Validator: &schema.Reference(
			Path: "vegetables",
			Qurey: query.MustParsePredicate(`type: {$in: ["broccoli", "carrot"]}`),
		},
	},

In validation of a reference value, Query.Match() would be called on the payload of the linked resource.

Implementation

We have some options on how to implement this:

1. Extend on the current ReferenceChecker interface to something like this:

type ReferenceChecker interface {
	ReferenceChecker(path string, query query.Predicate) FieldValidator
}

2. Replace the interface with something that can return an implementation of the new query.Resource interface.