How to specify a property can be null or a reference with swagger

Question

How to specify a property as null or a reference? discusses how to specify a property as null or a reference using jsonschema.

I'm looking to do the same thing with swagger.

To recap the answer to the above, with jsonschema, one could do this:

{
   "definitions": {
      "Foo": {
         # some complex object
      }
   },

   "type": "object",
   "properties": {
      "foo": {
         "oneOf": [
            {"$ref": "#/definitions/Foo"},
            {"type": "null"}
         ]
      }
   }
}

The key point to the answer was the use of oneOf.

The key points to my question:

1. I have a complex object which I want to keep DRY so I put it in a definitions section for reuse throughout my swagger spec: values of other properties; response objects, etc.

2. In various places in my spec a property may be a reference to such an object OR be null.

How do I specify this with Swagger which doesn't support oneOf or anyOf?

Note: some swagger implementations use x-nullable (or some-such) to specify a property value can be null, however, $ref replaces the object with what it references, so it would appear any use of x-nullable is ignored.

Answer 1

OpenAPI 3.1

Define the property as anyOf of the $ref and type: 'null'.

YAML version:

foo:
  anyOf:
    - type: 'null'   # Note the quotes around 'null'
    - $ref: '#/components/schemas/Foo'

JSON version:

"foo": {
    "anyOf": [
        { "type": "null" },
        { "$ref": "#/components/schemas/Foo" }
    ]
}

Why use anyOf and not oneOf? oneOf will fail validation if the referenced schema itself allows nulls, whereas anyOf will work.

OpenAPI 3.0

YAML version:

foo:
  nullable: true
  allOf:
  - $ref: '#/components/schemas/Foo'

JSON version:

"foo": {
    "nullable": true,
    "allOf": [
        { "$ref": "#/components/schemas/Foo" }
    ]
}

In OAS 3.0, wrapping $ref into allOf is needed to combine the $ref with other keywords - because $ref overwrites any sibling keywords. This is further discussed in the OpenAPI Specification repository: Reference objects don't combine well with “nullable”

Answer 2

For OpenAPI 3.0 for some reason using nullable: true followed by allOf didn't work for the OpenAPI interpreter I'm using. As a workaround I ended up defining a must-be-null ref called null_type that I can use in an anyOf construct.

Like so:

allOf:
  - anyOf:
      - $ref: "#/components/schemas/null_type"
      - $ref: "#/components/schemas/your_ref"
  - description: "optionally add other properties here..."

where:

schemas:
  null_type:
    title: "OpenAPI 3.0 null-type ref"
    description: "for adding nullability to a ref"
    enum: [null]

  your_ref:
    ...