Optional value with pattern

[careri]

Hi, I'm just getting started with schema validation. Nice concept, thanks!

I know I'm doing this wrong, but what would be the correct way of doing this?

I want a property resource_url
It can be valid in three different ways:

1. Null is allowed
2. Empty string is allowed
3. A string that matches the uri format.

This is my attempt, it doesn't work:

{
  "$id": "https://test.com/optional_value.json",
  "$schema": "https://json-schema.org/draft/2019-09/schema",
  "description": "Test",
  "properties": {
    "resource_url": {
      "description": "An optional string, should match the uri format if supplied"
    }
  },
  "type": "object",
  "anyOf": [
    {
      "if": {
        "properties": {
          "resource_url": {
            "type": "string",
            "minLength": 1
          }
        }
      },
      "then": {
        "properties": {
          "resource_url": {
            "format": "uri"
          }
        }
      }
    },
    {
      "if": {
        "properties": {
          "resource_url": {
            "type": "string",
            "maxLength": 0
          }
        }
      },
      "then": {
        "properties": {
          "resource_url": {
            "maxLength": 0
          }
        }
      }
    },
    {
      "if": {
        "properties": {
          "resource_url": {
            "type": "null"
          }
        }
      },
      "then": {
        "properties": {
          "resource_url": {
            "type": "null"
          }
        }
      }
    }
  ]
}

[jviotti]

Hi @careri , I think you can simplify your schema a lot by making use of anyOf instead of if/then/else. For example:

{
  "$id": "https://test.com/optional_value.json",
  "$schema": "https://json-schema.org/draft/2019-09/schema",
  "description": "Test",
  "properties": {
    "resource_url": {
      "description": "An optional string, should match the uri format if supplied",
      "anyOf": [
        { "type": "null" },
        { "const": "" },
        { "type": "string", "minLength": 1, "format": "uri" },
      ]
    }
  }
}

Though keep in mind that not all JSON Schema implementations validate format by default (some only report it as metadata, which we call "annotations" in JSON Schema), so double check that you get the results you expect (as an aside, I built a unit testing framework for schemas: https://github.com/sourcemeta/jsonschema/blob/main/docs/test.markdown), and if not, you can still force-validate the URI directly using pattern.

[careri]

Hi, and thank you! That looks a lot clearer, I knew I was doing something wrong.

That worked perfectly :)