rule not validated when property is not present at all

[flx-sta]

Describe the bug
rules with the schema {const: null} don't validate successfully when the respective property doesn't exist

To Reproduce
Steps to reproduce the behavior:

1. check out this repo on the specific branch: https://github.com/Felix-Puetz/jsonforms-vue-example/tree/bug/const-null-returns-true-on-empty-object
2. run npm install in it
3. run npm run serve in it
4. launch the given url in your browser

Example schema:

{
  type: "object",
  properties: {
    name: { type: "string" },
    mail: { type: "string" },
  },
  required: ["name", "mail"],
};

Example uischema:

{
  type: "VerticalLayout",
  elements: [
    {
      scope: "#/properties/name",
      type: "Control",
    },
    {
      scope: "#/properties/mail",
      type: "Control",
      rule: {
        condition: {
          scope: "#/properties/name",
          schema: {
              const: null
          },
        },
        effect: "DISABLE",
      },
    },
  ]
}

Expected behavior
mail field should be disabled when name is not present in data

Screenshots

Browser (please complete the following information):

• Browser all (latest release)

Used Setup (please complete the following information):
All info present in my repository

Additional context
spoke to @sdirix and he confirmed

[sdirix]

I would have expected the ui schema to work. It actually works when moving the scope of the rule one level up:

    {
      scope: "#/properties/mail",
      type: "Control",
      rule: {
        condition: {
          scope: "#",
          schema: {
            properties: {
              name: {
                const: null
              }
            },
          },
        },
        effect: "DISABLE",
      },
    },

We need to investigate whether this is a bug, according to spec, or some implementation defined behavior which we can adjust.

[lucas-koehler]

This is still reproducible with 3.7: Both the behavior from the issue and the comment above.
Debugging this, the value and schema are handed into ajv as expected.
I.e. undefined as value/data in the case described in the issue and { const: null } as the schema.
And the whole object as data in the alternative case and the schema from the condition.
Thus, this really seems to be evaluated differently by AJV.

Hunch (pending further investigation): AJV might treat an undefined data to validate as a special case

[lucas-koehler]

Investigating this further, this IMO works as expected according to JSON Schema specification:

A schema { "const": null } evaluates that a property has exactly the value null. However, null is not the same as an absent (or undefined) value. Thus, the behavior described in the issue is as expected.

The schema described in comment #1686 (comment) works for an absent name property because the name property in its condition schema is not required. Thus, the name property is not evaluated at all if it's absent and the validation succeeds. However, that would also be the case for any other const in the schema. Thus, this is not the recommended approach.

Enforcing an absent name property to succeed validation can be done with an inverted required using JSON schema's not property like this:

{
  scope: '#/properties/mail',
  type: 'Control',
  rule: {
    condition: {
      scope: '#',
      schema: {
        not: {
          required: ['name'],
        },
      },
    },
    effect: 'DISABLE',
  },
}

In conclusion, everything works as expected here and the rule schema was not chosen correctly for the intended outcome. Thus, I'm closing this.