Kit Kit - 1 month ago 32
JSON Question

How do I define a Json Schema containing definitions, in code

I am attempting to replicate the following

Json Schema
example, by defining the schema in code using
Newtonsoft.Json.Schema
:

{
"$schema": "http://json-schema.org/draft-04/schema#",

"definitions": {
"address": {
"type": "object",
"properties": {
"street_address": { "type": "string" },
"city": { "type": "string" },
"state": { "type": "string" }
},
"required": ["street_address", "city", "state"]
}
},

"type": "object",

"properties": {
"billing_address": { "$ref": "#/definitions/address" },
"shipping_address": { "$ref": "#/definitions/address" }
}


This is as close as I've got so far. (Example is in F# but might just as well be in C#.)

Code:

open Newtonsoft.Json.Schema
open Newtonsoft.Json.Linq

let makeSchema =
let addressSchema = JSchema()
addressSchema.Properties.Add("street_address", JSchema(Type = Nullable(JSchemaType.String)))
addressSchema.Properties.Add("city", JSchema(Type = Nullable(JSchemaType.String)))
addressSchema.Properties.Add("state", JSchema(Type = Nullable(JSchemaType.String)))
addressSchema.Required.Add "street_address"
addressSchema.Required.Add "city"
addressSchema.Required.Add "state"

let schema = JSchema()
schema.Properties.Add("billing_address", addressSchema)
schema.Properties.Add("shipping_address", addressSchema)
schema


Output:

{
"properties": {
"billing_address": {
"properties": {
"street_address": {
"type": "string"
},
"city": {
"type": "string"
},
"state": {
"type": "string"
}
},
"required": [
"street_address",
"city",
"state"
]
},
"shipping_address": {
"$ref": "#/properties/billing_address"
}
}
}


As you can see, only one of the two addresses is defined using a reference to another schema, and the address schema is in "properties" rather than "definitions". What's the trick to defining a schema in "definitions" and referencing it elsewhere?

Answer

Hackfest! :-)

According to the source code, JSON.NET Schema just doesn't write a definitions property, end of story. So it's all hopeless... Almost.

It does use the definitions property in another place, however. Namely - when generating schema from a type. During that process, it creates a JObject, pushes all schemas into it, and then adds that object to JSchema.ExtensionData under the definitions key. And when referencing a schema from another place, the schema writer will respect that definitions object, if present, thus making the whole thing work together.

So, armed with this knowledge, we can hack our way into it:

let makeSchema = 
    let addressSchema = JSchema()
    ...

    let definitions = JObject() :> JToken
    definitions.["address"] <- addressSchema |> JSchema.op_Implicit

    let schema = JSchema()
    schema.ExtensionData.["definitions"] <- definitions
    schema.Properties.Add("billing_address", addressSchema)
    schema.Properties.Add("shipping_address", addressSchema)
    schema

And voila! The resulting schema now has a definitions object, just as the sacred texts tell us it should:

{
  "definitions": {
    "address": {
      "properties": {
        "street_address": {
          "type": "string"
        },
        "city": {
          "type": "string"
        },
        "state": {
          "type": "string"
        }
      },
      "required": [
        "street_address",
        "city",
        "state"
      ]
    }
  },
  "properties": {
    "billing_address": {
      "$ref": "#/definitions/address"
    },
    "shipping_address": {
      "$ref": "#/definitions/address"
    }
  }
}

A few notes:

  1. The definitions name is not special from the JSON.NET's point of view. If you change the line schema.ExtensionData.["definitions"] to something different, say schema.ExtensionData.["xyz"], it will still work, with references all pointing to "#/xyz/address".
  2. This whole mechanism, obviously, is a hack Apparently not, according to James Netwon-King. The key insight seems to be that the JsonSchemaWriter will be able to lookup any previous mentions of schemas and use references to them in other places. This allows one to put shove schemas wherever one likes and expect them to be referenced.
  3. That op_Implicit call in there is necessary. JSchema is not a subtype of JToken, so you can just jam it into definitions.["address"] like that, you have to convert it to JToken first. Fortunately, there is an implicit cast operator defined for that. Unfortunately, it's not straightforward, there seems to be some magic going on. This happens transparently in C# (because, you know, there is not enough confusion as it is), but in F# you have to call it explicitly.