How to write a JSON schema for array of objects?

Question

My JSON string would be formatted as:

{
    "count":3,
    "data":[
        {
            "a":{"ax":1}
        },
        {
            "b":{"bx":2}
        },
        {
            "c":{"cx":4}
        }
    ]
}

The data array contains many a and b and c. And no other kinds of objects.

If count==0, data should be an empty array [].

I'm using https://github.com/hoxworth/json-schema to validate such JSON objects in Ruby.

require 'rubygems'
require 'json-schema'

p JSON::Validator.fully_validate('schema.json',"test.json")

The schema.json is:

{
  "type":"object",
  "$schema": "http://json-schema.org/draft-03/schema",
  "required":true,
  "properties":{
     "count": { "type":"number", "id": "count", "required":true },
     "data": { "type":"array", "id": "data", "required":true,
       "items":[
           { "type":"object", "required":false, "properties":{ "a": { "type":"object", "id": "a", "required":true, "properties":{ "ax": { "type":"number", "id": "ax", "required":true } } } } },
           { "type":"object",  "required":false, "properties":{ "b": { "type":"object", "id": "b", "required":true, "properties":{ "bx": { "type":"number", "id": "bx", "required":true } } } } },
           { "type":"object",  "required":false, "properties":{ "c": { "type":"object", "id": "c", "required":true, "properties":{ "cx": { "type":"number", "id": "cx", "required":true } } } } }
       ]
     }
  }
}

But this for test.json will pass the validation while I suppose it should fail:

{
  "count":3,
  "data":[
      {
          "a":{"ax":1}
      },
      {
          "b":{"bx":2}
      },
      {
          "c":{"cx":2}
      },
      {
          "c": {"z":"aa"}
      }
   ]
}

And this as test.json will fail, while I suppose it should pass:

{
  "count":3,
  "data":[
      {
          "a":{"ax":1}
      },
      {
          "b":{"bx":2}
      }
   ]
}

Seems the wrong schema is validating that the data array contains a,b,c once.

What the right schema should be?

@Phrogz I have updated the question. You can see the example. — Green Su
– Green Su, Commented May 30, 2012 at 7:01

Community · Accepted Answer · 2021-10-07 07:13:45Z

29

From the JSON schema spec, section 5.5. items:

When this attribute value is an array of schemas and the instance
value is an array, each position in the instance array MUST conform
to the schema in the corresponding position for this array. This
called tuple typing.

Your schema definition requires the first three elements of the array to be exactly those 'a', 'b' and 'c' elements. If items is left empty, any array element is allowed. Similarly, if additionalItems is left empty, any additional array element is allowed.

To get what you want, you need to specify "additionalItems": false and for the items, I think the following (somewhat shortened from your definitions) should work:

"items": {
  "type": [
     {"type":"object", "properties": {"a": {"type": "object", "properties": {"ax": { "type":"number"}}}}},
     {"type":"object", "properties": {"b": {"type": "object", "properties": {"bx": { "type":"number"}}}}},
     {"type":"object", "properties": {"c": {"type": "object", "properties": {"cx": { "type":"number"}}}}}
  ]
}

edited Oct 7, 2021 at 7:13

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answered May 30, 2012 at 8:03

Confusion

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2 Comments

Mike Marcacci Over a year ago

Is this the correct way to use "type" here? Wouldn't the value of "items" be the array instead?

jtimz Over a year ago

@Confusion, sorry for opening this again, but the proposed solution doesn't work; so for example should I have more than 3 of the proposed objects in data array then the schema validation fails... any idea how to solve this?

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