Unclear about the meaning of propertyNames

Question

Here is a JSON schema that uses propertyNames:

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "type": "object",
    "propertyNames": {"enum": ["num", "name"]}
}

I "think" the meaning of that schema is this: A conforming JSON instance must be an object and the object must contain a "num" property and a "name" property; the value of the two properties is unspecified. Is that correct, is that what the schema means?

I created this instance:

{
   "num": 10
}

I validated that instance against the schema and the validator says it is valid. Hmm, why? Doesn't the schema specify that the object must contain both "num" and "name"?

Answer 1

propertyNames is a schema that all of an object's properties must be valid against. Let's take a look at less confusing example.

{
  "type": "object",
  "propertyNames": { "maxLength": 3, "minLength": 3 },
  "patternProperties": {
    "": { "type": "number" }
  }
}

This describes an object where all properties names must be of length 3 and all property values must be numbers. Here's an example.

{
  "usd": 1,
  "eur": 0.86815,
  "gbp": 0.76504,
  "cad": "1.31004",  <= Invalid property value
  "xx": 1.11         <= Invalid property name
}

Going back to your example, there is one property, "num", which valid against the propertyNames schema { "enum": ["num", "name"] }. Therefore, the value in your example is valid.