nullable fields and reference object

[timothyqiu]

Describe the bug
If two fields in an object both reference the same schema component, and the referenced schema contains a nullable field, prism rejects the documents with Type ["string","null"] is not a valid type.

The behavior is also inconsistent between using a schema reference and writing the schema directly.

To Reproduce

1. Given this OpenAPI document

   openapi: 3.0.0
   
   paths:
     /:
       get:
         responses:
           '200':
             content:
               application/json:
                 schema:
                   type: object
                   properties:
                     first:
                       $ref: '#/components/schemas/TestSchema'
                     second:
                       $ref: '#/components/schemas/TestSchema'
   
   components:
     schemas:
       TestSchema:
         type: object
         properties:
           output:
             type: string
             nullable: true

2. Run this CLI command prism mock schema.yml
3. See error ✖ fatal Type ["string","null"] is not a valid type

If the sub-schema is used directly instead of using a reference schema, it works fine:

openapi: 3.0.0

paths:
  /:
    get:
      responses:
        '200':
          content:
            application/json:
              schema:
                type: object
                properties:
                  first:
                    type: object
                    properties:
                      output:
                        type: string
                        nullable: true
                  second:
                    type: object
                    properties:
                      output:
                        type: string
                        nullable: true

Expected behavior
No error is reported. Using a schema reference works the same as writing the schema directly.

Environment:

• Library version: 3.0.4
• OS: macOS Mojave 10.14.6

[XVincentX]

Ok, verified. This is happening because our resolver is caching the references — trying to re-transform a schema that has been already transformed. We need to clone the schema.

Once stoplightio/http-spec#18 and propagated here in Prism, this will be fixed. Thanks!