Referencing Instance Data

Most of the questions that center around referencing data involve comparing the values of properties. This typically comes in a few flavors:

• How do I require that property A is less than property B?
• How do I require that property A is one of a list of things specified by property B?
• How do I require that…

Requiring that A < B

We want to specify a minimum for B that is the value of A. Unfortunately, JSON Schema doesn’t have a mechanism that allows this; it only has minimum which must be a static (unchanging) value. Specifically, as enforced by the meta-schema, minimum must be a number.

{
  "type": "object",
  "properties": {
    "A": { "type": "number" },
    "B": {
      "type": "number",
      "minimum": ???
    }
  }
}

We can’t put a number here because we want that value to depend on the value of A.

To solve this, we remove the minimum keyword and add the data keyword. Inside data, we specify minimum with a plain JSON Pointer (not URI-encoded) that points to where in the instance we want to get the value; in this case, from the A property.

{
  "$schema": "https://json-everything.net/meta/data-2022",
  "type": "object",
  "properties": {
    "A": { "type": "number" },
    "B": {
      "type": "number",
      "data": {
        "minimum": "/A"
      }
    }
  }
}

Passes	Fails
{ "A": 5, "B": 10 }	{ "A": 15, "B": 10 }

Requiring that A is one of the items in B

We want to specify an enum for A, and the values in that enum are contained in B. Similarly to the numbers problem above, JSON Schema only allows for explicit lists for enum, so we can’t source it from somewhere else.

{
  "type": "object",
  "properties": {
    "A": { "enum": [ ??? ] },
    "B": {
      "type": "array",
      "items": { "type": "string" }
    }
  }
}

We know that B must be an array of strings, but how do we define that A has to be one of the strings defined in B?

Again, we need to define the keyword that needs the reference (enum) inside a data keyword and provide a plain JSON Pointer that points to where in the instance we want to get the values; in this case from the B property.

{
  "$schema": "https://json-everything.net/meta/data-2022",
  "type": "object",
  "properties": {
    "A": {
      "data": {
        "enum": "/B"
      }
    },
    "B": {
      "type": "array",
      "items": { "type": "string" }
    }
  }
}

Passes	Fails
{ "A": "cat", "B": [ "dog", "cat", "gerbil" ] }	{ "A": "giraffe", "B": [ "dog", "cat", "gerbil" ] }