# schema-no-contradictions

Source: https://apigenie.pl/docs/linter/openapi/rules/schema-no-contradictions

This rule reports schemas with contradicting constraints, which reject every value or
some of the properties they describe.

## Rule Details

The rule merges each schema with its `allOf` members, following references, and reports
contradictions it can prove:

- `allOf` members requiring types with nothing in common, such as `string` and `integer`;
- bounds excluding every value: `minimum` above `maximum` (taking `exclusiveMinimum` and
  `exclusiveMaximum` into account, and integers when the type is `integer`), `minLength`
  above `maxLength`, `minItems` above `maxItems`, `minProperties` above `maxProperties`,
  or more required properties than `maxProperties`;
- an `allOf` member rejecting every value, e.g. `false` or `not: {}`, and `anyOf` or
  `oneOf` whose every branch does;
- `enum` or `const` values of an `allOf` member none of which satisfies the whole schema;
- a required property whose schema rejects every value, or which an `allOf` member with
  `additionalProperties: false` does not declare;
- a property which can never be present because an `allOf` member with
  `additionalProperties: false` does not declare it. This is the common mistake of
  closing a base schema and extending it with `allOf`; use `unevaluatedProperties: false`
  (OpenAPI 3.1) on the combined schema instead.

A contradiction is reported once, at the schema it originates from, and not again at
every schema using that one. Formats are not checked, and constraints the rule cannot
reason about (such as `pattern`) never lead to a report.

## Configuration

None.

## Examples

**Good**. `Cat` declares every property it allows.

```yaml
components:
  schemas:
    Pet:
      type: object
      required: [petType]
      properties:
        petType:
          type: string
    Cat:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - properties:
            meow:
              type: boolean
      unevaluatedProperties: false
```

**Bad**. `Pet` forbids `meow`, so a `Cat` can never have it.

```yaml
components:
  schemas:
    Pet:
      type: object
      additionalProperties: false
      required: [petType]
      properties:
        petType:
          type: string
    Cat:
      allOf:
        - $ref: '#/components/schemas/Pet'
        - properties:
            meow:
              type: boolean
```

**Bad**. No value is both a string and an integer.

```yaml
components:
  schemas:
    Id:
      allOf:
        - type: string
        - type: integer
```

## When Not to Use It

We recommend to always enable this rule.

## Compatibility

This rule is compatible with all OpenAPI 3.x versions. In OpenAPI 3.0 documents,
`nullable` and the boolean `exclusiveMinimum`/`exclusiveMaximum` are taken into account,
and keywords beside `$ref` are ignored as the specification requires.