JSONSchema - Required property dependent on parent property

Question

I would like to apply an additional "required" property in an array sub schema based on the presence of a property in the root schema. I have my schema set like this:

{
    "$schema": "http://json-schema.org/draft-07/schema#",
    "type": "object",
    "required":[
       "isParentDependency",
       "subArray"
    ],
    "properties": {
        "isParentDependency": {
            "$id": "#/properties/isParentDependency",
            "type": "boolean"
        },
        "subArray": {
            "$id": "#/properties/subArray",
            "type": "array",
            "items": {
                "$id": "#/properties/subArrayItem",
                "required": ["alwaysRequiredProp"],
                "dependencies": {
                    "isParentDependency":{
                        "required":["requiredPropIfIsParentDependency"]
                    }
                },
                "properties": {
                    "alwaysRequiredProp": {
                        "$id": "#/properties/subArray/items/properties/alwaysRequiredProp",
                        "type": "boolean"
                    },
                    "requiredPropIfIsParentDependency": {
                        "$id": "#/properties/subArray/items/properties/requiredPropIfIsParentDependency",
                        "type": "boolean"
                    }
                }
            }
        }
    }
}

Passing Cases

{
    "isParentDependency": false,
    "subArray": [{
        "alwaysRequiredProp": true
    }]
}

    "isParentDependency": true,
    "subArray": [{
        "alwaysRequiredProp": true,
        "requiredPropIfIsParentDependency":true
    }]
}

Failing Cases

{
    "isParentDependency": true,
    "subArray": [{
        "alwaysRequiredProp": true
    }]
}

Clearly this is not going to work but I have been unable to figure out how to make a pointer to the root schema (or apply an if/else type solution with a $ref)

Any guidance greatly appreciated!

Answer 1

With JSON Schema, each level of nested properties down the tree cannot see up the tree. Therefore you have to define your conditions at the top most level to which it needs to see. In this case, that's the root level.

Take a look at this schema.

{
  "$schema": "http://json-schema.org/draft-07/schema#",
  "type": "object",
  "definitions": {
    "subArray": {
      "type": "array",
      "items": {
        "required": [
          "alwaysRequiredProp"
        ],
        "properties": {
          "alwaysRequiredProp": {
            "type": "boolean"
          },
          "requiredPropIfIsParentDependency": {
            "type": "boolean"
          }
        }
      }
    }
  },
  "required": [
    "isParentDependency",
    "subArray"
  ],
  "properties": {
    "isParentDependency": {
      "type": "boolean"
    },
    "subArray": {
      "$ref": "#/definitions/subArray"
    }
  },
  "if": {
    "properties": {
      "isParentDependency": {
        "const": true
      }
    }
  },
  "then": {
    "properties": {
      "subArray": {
        "items": {
          "required": [
            "requiredPropIfIsParentDependency"
          ]
        }
      }
    }
  }
}

subArray has been moved to a definition and then referenced, so the conditional application logic can be seen easier.

The definition of subArray only determines the properties of the items structure if they are provided, and requires alwaysRequiredProp. It does not require requiredPropIfIsParentDependency.

The conditional application comes in using the if and then keywords.

If the if schema is true, then the else schema is applied.

The value of if is a schema. It is applicable to the root level. It requires isParentDependency be true (it does not require the property itself, because that is ALWAYS required. Requiring the property inside the if statement would NOT mean the property is always required, unless else was false, but we're not even using else here).

When isParentDependency is true, the then schema is applied, which looks down the tree to subArray's items, requiring requiredPropIfIsParentDependency.

You can test this working in browser using jsonschema.dev (preloaded with this schema and your fail instance).