Check for incorrect discriminator use

[adamaltman]

Is your feature request related to a problem? Please describe.

Yes: users need to get feedback about incorrect discriminator use.

Describe the solution you'd like

no-discriminator-property-in-object

Problem message:

The discriminator property {insert the property name} must exist in schemas {...names}.

Describe alternatives you've considered None.

Additional context Here is a minimal example of a problematic schema:

components:
  schemas:
    Problematic:
      type: object
      properties:
        data:
          discriminator:
            propertyName: objectType
          oneOf:
          - $ref: '#/components/schemas/A'
          - $ref: '#/components/schemas/B'
        objectType:
          type: string
    A:
      type: object
      properties:
        message:
          type: string
    B:
      type: object
      properties:
        message:
          type: string

Notice that objectType is defined in the Problematic schema and not in A and B, and this is the problem that it is not defined in A and B.

[TylerRick]

I tried out the example and it does give an error already:

Example validation errored: discriminator: oneOf subschemas (or referenced schemas) must have "properties/objectType" or match mapping.

23 | application/json:
24 |   schema:
25 |     $ref: '#/components/schemas/Problematic'

Is that error not exactly like (or sufficiently close enough to) the one requested by this issue?

[SmoliyY]

@TylerRick you probably add example for this schema and this error was generated by the no-invalid-media-type-examples rule. So this is kind of different error. But thank you that you noticed that, because it can help and simplify implementation of proposed rule in future!

[TylerRick]

Ah yes, you are right. I've been so focused on examples lately that I didn't notice this issue was not about an example rule... :smile: