How to apply constraint on the basis of count of properties?

[girish-tharwani]

Hello experts,

I am trying to write a schema that will validate json of this type

{
    "Steps": {
        "0": {
            "something": "something"
        },
        "1": {
            "something": "something"
        }
    }
}

with constraints:
1: Steps can be empty ie {"Steps":{}} is ok
2: Key values can be 0 to 99.
3: if Steps is not empty, it must have key 0 in it.

I have used patternProperties to apply the constraint on key names (constraint 2)

{
    "Steps": {
        "patternProperties": {
            "^[0-9]$|^[1-9][0-9]$": {
                "type": "object",
                "$ref": "#/$defs/StepDetails"
            }
        },
        "additionalProperties": false
    }
}

But I dont know how to apply the constraint number 3 above. I don't want to make property "0" required unconditionally because that will interfere with my constraint number 1 so how can I specify that "if count of properties in Steps >0, property "0" is required"

Thanks,
GT

[girish-tharwani]

This seem to work but I was hoping for a more elegant If-then solution

{
    "Steps": {
        "patternProperties": {
            "^[0-9]$|^[1-9][0-9]$": {
                "type": "object",
                "$ref": "#/$defs/StepDetails"
            }
        },
        "additionalProperties": false,
        "oneOf": [
            {
                "required": [
                    "0"
                ]
            },
            {
                "maxProperties": 0
            }
        ]
    }
}

[gregsdennis]

I think this is a nice solution, actually. It's easy to understand what it's doing.

One thing you will need to do is wrap the "Steps" in a properties keyword.

{
  "type": "object",
  "properties": {
    "Steps": { ... }
  }
}

[girish-tharwani]

Yes, Steps is defined like that in the schema. This was just a snippet so some portions were left out....

[jdesrosiers]

You can use minProperties to do a conditional on the number of properties present.

"if": { "minProperties": 1 },
"then": { "required": ["0"] }

[girish-tharwani]

tried this. This does not work. I think "minProperties" defines the constraint of minimum number of properties required. It does not maintain the count of properties present.

[jdesrosiers]

It works. It expresses the constraint, "if there is at least one property (minProperties: 1), then the '0' property is required".

If this doesn't work for you, the validator you're using probably doesn't support the if/then keywords.

"Steps": {
  "type": "object",
  "patternProperties": {
    "^[0-9]$|^[1-9][0-9]$": { "type": "object" }
  },
  "additionalProperties": false,

  "if": { "minProperties": 1 },
  "then": { "required": [ "0" ] }
}

[girish-tharwani]

tried again and it works! Must have done something wrong last time. Thanks.