Can JSON Schema require one value equal another?

Question

consider the following schema:

{
  "properties":{
    "a": { "type":"string"},
    "meta": {
      "type": "object",
      "properties": {
        "a": { /*what might I put here to require that obj.a == obj.meta.a?*/ }
      }
    }
  }
}

Can I require a == meta.a somehow?

Totally not required, and not even applicable to the case that led me to this question, but it'd also be interesting to know if I could do any operations, like substrings, or exponentiation (if a had been a number).

Answer 1

Out of the box, no. JSON Schema doesn't define a way to reference parts of the instance, which means that it can't do those kinds of comparisons.

However, I have a data vocab that you might be able to use if you're in .Net. I don't think any other languages support it yet.

Basically, you'd have

{
  "$schema": "https://json-everything.net/meta/data-2023",
  "properties": {
    "a": { "type":"string" },
    "meta": {
      "type": "object",
      "properties": {
        "a": {
          "data": {
            "const": "/a"
          }
        }
      }
    }
  }
}

For this schema,

{
  "a": "string",
  "meta": {
    "a": "string"
  }
}

passes and

{
  "a": "string",
  "meta": {
    "a": "also string"
  }
}

fails.

What's happening is that the data keyword is building a schema by using its properties as keywords and dereferencing the values. Plain JSON Pointers are resolved against the instance, so this effectively builds

{
  "const": "<whatever "a" is>"
}

in memory and evaluates meta.a against that.

You can test this at my playground https://json-everything.net/json-schema