Implementing a constraint inside $def using the field value from a higher level schema

Question

Below is a simplified / sanitized view of my actual schema. Is it possible to implement a constraint on fieldD (lets say a constraint on array size) using the value of fieldA?

{
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
        "fieldA": {
            "type": "boolean"
        },
        "fieldB": {
            "type": "object",
            "$ref": "#/$defs/fieldB"
        }
    },
    "$defs": {
        "fieldB": {
            "type": "object",
            "properties": {
                "fieldC": {
                    "type": "string"
                },
                "fieldD": {
                    "$ref": "#/$defs/fieldD"
                }
            }
        },
        "fieldD": {
            "type": "array"
        }
    }
}

Answer 1

You can use if, then syntax to check conditions on fieldA and apply constraints on fieldD

{
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
        "fieldA": {
            "type": "boolean"
        },
        "fieldB": {
            "type": "object",
            "$ref": "#/$defs/fieldB"
        }
    },
    "$defs": {
        "fieldB": {
            "type": "object",
            "properties": {
                "fieldC": {
                    "type": "string"
                },
                "fieldD": {
                    "$ref": "#/$defs/fieldD"
                }
            }
        },
        "fieldD": {
            "type": "array"
        }
    },
    "if": {
        "required": ["fieldA"],
        "properties": {
            "fieldA": {
                "const": true
            }
        }
    },
    "then": {
        "properties": {
            "fieldB": {
                "properties": {
                    "fieldD": {
                        "minItems": 1
                    }
                }
            }
        }
    },
    "else": {
        "properties": {
            "fieldB": {
                "properties": {
                    "fieldD": {
                       <some other constraint or omit `else` completely for a default behavior>                    }
                }
            }
        }
    }
}