How to specify that a JSON instance is defined by a specific JSON Schema

Question

My question is how can I know which JSON schema to use to validate a particular JSON document? I specified the URL to the schema in the id field from it. Is this enough? Should I put the id in the JSON? I am not sure I understand how to connect a JSON to a specific JSON Schema.

Here is my schema

{
 "$schema": "http://json-schema.org/draft-04/schema#",
 "id": "url/schema.json",
 "title": "title",
 "definitions": {
    "emailObject": {
        "type": "object",
        "properties":{
            "name": {
                "description": "The name of the customer",
                "type": "string",
                "maxLength": 200
            },
            "email": {
                "description": "The email of the customer",
                "type": "string",
                "format": "email",
                "maxLength": 100
            }
        }
    }
 }

Answer 1

To add to and clarify Tom's answer, here is an example of how you can link a JSON document to a JSON Schema. There is no standard way of doing this outside the context of an HTTP response. If that is something you need, you will have to come up with your own strategy.

GET /data/my-data HTTP/1.1

HTTP/1.1 200 OK
Content-Type: application/json
Link: </schema/my-schema> rel=describedby

{ "name": "Fake Fakerson", "email": "fake@fakerson.com" }

GET /schema/my-schema HTTP/1.1

HTTP/1.1 200 OK
Content-Type: application/schema+json

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "id": "url/schema.json",
  "title": "title",
  "definitions": {
    "emailObject": {
      "type": "object",
      "properties":{
        "name": {
          "description": "The name of the customer",
          "type": "string",
          "maxLength": 200
        },
        "email": {
          "description": "The email of the customer",
          "type": "string",
          "format": "email",
          "maxLength": 100
        }
      }
    }
  }
}

Answer 2

According to 10.1 of the specification:

It is RECOMMENDED that instances described by a schema/profile provide a link to a downloadable JSON Schema using the link relation "describedby", as defined by Linked Data Protocol 1.0, section 8.1 [W3C.REC-ldp-20150226]. (emphasis mine)

This would appear to describe exactly the behaviour you require, however, a casual perusal of the Linked Data Protocol section 8.1 leaves us none the wiser:

The relationship A describedby B asserts that resource B provides a description of resource A. There are no constraints on the format or representation of either A or B, neither are there any further constraints on either resource (emphasis mine)

After a quick google search, I found this question, which at first glance would appear to be duplicated by your question. However, upon deeper inspection, the question is actually about inheritance within schemas, not the referencing of a schema from it's supported instances.

One of the answers, rather intriguingly, provides a solution which draws on the JSON-Hyper-schema standard - an attempt to extend the JSON-schema standard to support the definition of application-level semantics.

The way it achieves this is by use of the links collection:

{
   ...
   "links":[
      {
         "rel":"describedby",
         "href":"{+fileType}"
      }
   ]
}

It turns out that this is based on another standard RFC5988 - Web Linking which happens to be the same standard which allows us to load CSS into HTML pages.

As @Jason points out in his comment -

Your first quote, the one from the spec, is the right way to do it. The linked data definition of describedby does not contradict the JSON Schema spec. It's a purposefully broad definition so it can be applied to any media type that describes data. That includes JSON Schema, XML Schema, or anything else.

So, it would appear that including a links collection in your schema instance would be the correct way to reference the schema. So in your specific case, you could do this:

{
   ...
   "links":[
      {
         "rel":"describedby",
         "href":"url/schema.json" // I assume!!
      }
   ]
}

Even though this may be correct, I don't know how many JSON parsers will respect this when resolving to an actual schema via the link.