How to $ref an object from within an array in the same JSON file

Question

I would like to declare and define an array of my own objects but I am having problems validating the instance the way I expect.

My $ref points to an object in the same schema (as specified here you can only point to a schema; Can JSON integer attributes be referenced?)

I have followed this link for guidance; https://spacetelescope.github.io/understanding-json-schema/structuring.html

I am validating here; http://json-schema-validator.herokuapp.com/

I would like to make the schema stricter by not allowing further elements of other kinds in the feeder_tx array but can't seem to get the syntax right.

This is the schema:

{
    "id": "params-schema",
    "$schema": "http://json-schema.org/draft-04/schema#",
    "description": "my schema",

    "definitions": {

        "tx": {

            "type": "object",
            "properties": {

                "comment": {"type": "string"},

                "max_channels": {

                    "type": "integer",
                    "minimum" : 0,
                    "maximum" : 10000,
                    "additionalProperties": false
                }                
            },

            "additionalProperties": false,
            "required": ["max_channels"]
        }
    },

    "type": "object",
    "properties": {

        "feeder_tx": {

            "type": "array",
            "minItems": 1,
            "maxItems": 4,
            "items": {

                "type": "object",
                "properties": {

                    "comment": {"type": "string"},

                    "info": {

                        "$ref": "#/definitions/tx",
                        "additionalProperties": false
                    }
                }
            }
        }
    },

    "required": ["feeder_tx"]
}

This is the instance:

{
    "feeder_tx": [

            {"lala": 25, "max_channels": 1499}
    ]
}

Which I would like to fail because of the addition of lala but instead it validates successfully.

If I add an "additionalProperties": false at the end of the "properties" section of items the validator complains about both "lala" and "max_channels".

This makes sense because the next level is "info"

[ {
  "level" : "error",
  "schema" : {
    "loadingURI" : "#",
    "pointer" : "/properties/feeder_tx/items"
  },
  "instance" : {
    "pointer" : "/feeder_tx/0"
  },
  "domain" : "validation",
  "keyword" : "additionalProperties",
  "message" : "object instance has properties which are not allowed by the schema: [\"lala\",\"max_channels\"]",
  "unwanted" : [ "lala", "max_channels" ]
} ]

If I try to make the instance refer to "info" the following error occurs:

[ {
  "level" : "fatal",
  "message" : "URI \"params-schema#\" is not absolute",
  "uri" : "params-schema#",
  "info" : "other messages follow (if any)"
} ]

for this instance data:

{

    "feeder_tx": [

            {"info": { "max_channels": 1499}}
    ]

}

In fact, I do not see why I need to have a feeder_tx/items/info object as the thing I am $refing is an object.

So I revert the instance data back and remove this object. The following error occurs:

[ {
  "level" : "fatal",
  "message" : "URI \"params-schema#\" is not absolute",
  "uri" : "params-schema#",
  "info" : "other messages follow (if any)"
} ]

I.e the schema and instance becomes this:

"type": "object",
"properties": {

    "feeder_tx": {

        "type": "array",
        "minItems": 1,
        "maxItems": 4,
        "items": {

            "$ref": "#/definitions/tx"
        }
    }
},

Can someone please explain what is happening here, and the correct way to do it?

Restructuring the schema to not use $ref can solve the problem but I would like to know how to do it with references.

{
    "id": "params-schema",
    "$schema": "http://json-schema.org/draft-04/schema#",
    "description": "parameters schema",


    "type": "object",
    "properties": {

        "feeder_tx": {

            "type": "array",
            "minItems": 1,
            "maxItems": 4,
             "items": {

            "type": "object",
            "properties": {

                "comment": {"type": "string"},

                "max_channels": {

                    "type": "integer",
                    "minimum" : 0,
                    "maximum" : 10000,
                    "additionalProperties": false
                }                
            },

            "additionalProperties": false,
            "required": ["max_channels"]
            }
        }
    },

    "required": ["feeder_tx"]
}

gives the correct validation error:

[ {
  "level" : "error",
  "schema" : {
    "loadingURI" : "#",
    "pointer" : "/properties/feeder_tx/items"
  },
  "instance" : {
    "pointer" : "/feeder_tx/0"
  },
  "domain" : "validation",
  "keyword" : "additionalProperties",
  "message" : "object instance has properties which are not allowed by the schema: [\"lala\"]",
  "unwanted" : [ "lala" ]
} ] 

Thanks.

Answer 1

This works:

{
  "id": "#",
  "$schema": "http://json-schema.org/draft-04/schema#",
  "description": "parameters schema",
  "definitions": {
    "item": {
      "type": "object",
      "properties": {
        "comment": {
          "type": "string"
        },
        "max_channels": {
          "type": "integer",
          "minimum": 0,
          "maximum": 10000,
          "additionalProperties": false
        }
      },
      "additionalProperties": false,
      "required": [
        "max_channels"
      ]
    }
  },
  "type": "object",
  "properties": {
    "feeder_tx": {
      "type": "array",
      "minItems": 1,
      "maxItems": 4,
      "items": {
        "$ref": "#/definitions/item"
      }
    }
  },
  "required": [
    "feeder_tx"
  ]
}

Tried using:

• http://www.jsonschemavalidator.net/
• https://jsonschemalint.com/#/version/draft-04/markup/json
• https://json-schema-validator.herokuapp.com/