JSON schema to enforce array contents

Question

Hi all and thanks in advance.

I am attempting to create a JSON schema to enforce an array to contain one A and B object and N C objects, where A and B are C objects and N is an integer inclusively between 0 and infinity.

Therefor :

[A, B] [A, B, C1] [A, B, C1, .., CN]

Are all valid, though :

[A] [A, C1] [A, C1, .., CN]

Are not valid.

To make clear, A and B must be present. C objects are optional, though you may have as many as you would like.

C object schemas :

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

  "type": "object",
  "required": ["id", "name"],

  "properties": {
    "id": {
      "type": "integer"
    },
    "name": {
      "type": "string"
    }
  },
  "additionalProperties": false
}

So a C object is any valid JSON object containing only the properties "id" and "name" where "id" is an integer and "name" is a string.

A and B object schemas :

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

  "type": "object",
  "required": ["id", "name"],

  "properties": {
    "id": {
      "type": "integer"
    },
    "name": {
      "type": "string",
      "enum": ["A"]
    }
  },
  "additionalProperties": false
}

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

  "type": "object",
  "required": ["id", "name"],

  "properties": {
    "id": {
      "type": "integer"
    },
    "name": {
      "type": "string",
      "enum": ["B"]
    }
  },
  "additionalProperties": false
}

A and B objects differ from C objects in that there name value is enforced. The name value of an A object must be a value contained in the field enum, where enum contains a single value.

My most complete schema to date is :

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "title": "To Date Solution",
  "description": "So far this is the most complete attempt at enforcing values to be contained within a JSON structure using JSON schemas.",

  "type": "array"
  "items": {
    "allOf": [
      {
        "$schema": "http://json-schema.org/draft-04/schema#",
        "title": "C Object",

        "type": "object",
        "required": ["id", "name"],

        "properties": {
          "id": {
            "type": "integer"
          },
          "name": {
            "type": "string"
          }
        },
        "additionalProperties": false
      }
    ]
  }
}

This enforces that all objects contained within must be of type C, which A and B are, though I am unsure how to enforce that at least single instance of A and B are contained within my array.

Answer 1

What you are looking for is called "tuple typing".

If the value of the items keyword is an array, then the items in the array data must match up with the schema in the corresponding position. Additional items (past the last index) are matched by additionalItems (or are disallowed if additionalItems is false).

So, roughly what you want is something like:

{
    "type": "array",
    "items": [
        {"$ref": "#/definitions/itemTypeA"},
        {"$ref": "#/definitions/itemTypeB"}
    ],
    "additionalItems": {"$ref": "#/definitions/itemTypeC"},
    "definitions": {
        ... actual definitions for A/B/C ...
    }
}

If you want to ensure that A and B exist, then you simply specify a minimum length using minItems, so there are at least two items (which because of "tuple typing", must match up with A and B).

(This also assumes that A and B are the first items in the array. If that's not what you want, then it gets a bit more complicated - although there is a contains keyword proposed for v5 that would handle that neatly.)

Slightly more detailed live demo:

• Valid: [A, B]
• Invalid: [A, C]
• Valid: [A, B, C, C]