Merge/Patch for schema with inner ref question

Hi,

I've started using JSON schema in a project and it's all working well until I've tried using merge patch. There doesn't seem to be too many examples out there regarding merge/patch for cases where the schema being modified has references inside it so I was hoping I could get some guidance. I have a test schema:

const Ajv = require("ajv")
const ajv = new Ajv({allErrors: true, v5: true})
ajv.addMetaSchema(require('ajv/lib/refs/json-schema-draft-06.json'));
require('ajv-merge-patch')(ajv);

console.log("Start with base schema ================================")

const a = {
  "$id": "a.json#",
  type: "object",
  properties: {
    size: {type: "number"}
  },
  required: ["size"],
  additionalProperties: false
}
ajv.addSchema(a, "a.json")

A simple merge works okay

console.log("Simple Merge Test =====================================")
const b = {
  "$id": "b.json#",
  $merge: {
    source: { "$ref": "a.json#" },
    with: {
      properties: {
        length: { type: "number" }
      },
      required: ["length"],
      additionalProperties: false
    }
  }
}
const bv = ajv.compile(b);
console.log(bv({ length: 3, size: 4}))

however, if I create a schema with a reference to the first one inside and try to extend the inner schema via the outer schema it fails:

const c = {
  "$id": "c.json#",
  type: "object",
  properties: {
    inner: { "$ref": "a.json#" }
  }
}

console.log("Nested Ref Merge test ==================================")
const d = {
  "$id": "d.json#",
  $merge: {
    source: { "$ref": "c.json#" },
    with: {
      properties: {
        inner: {
          properties: {
            length: { type: "number" }
          },
          required: ["length"]
        }
      }
    }
  }
}
const dv = ajv.compile(d)

I believe this is due to refs not being replaced in the schema. Firstly I was wondering if what I am doing is odd or not - the main reason I was doing it this way is because in my actual scenario I have multiple properties based off of the same schema but only want to merge additional properties into some of them.

If it's not odd then how would I get the above to work?

Many thanks

ajv-validator / ajv-merge-patch

Merge/Patch for schema with inner ref question #20