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

Not easy to do that. Even almost impossible. Your options :

Wait

There is a very long discussion about this point, maybe one day it will be done...

Use vendors extensions

You can use vendors extensions like x-oneOf and x-anyOf. I have already taken this hard way: You must to upgrade all used 'swagger tools' to take into account these vendors extensions.

In my case, we needed 'only' to :

• Develops our own Jax-RS parser with customized annotations in order to extract swagger API file from sources
• Extends swagger-codegen to take into account these extensions to generate java code for our clients
• Develops our own swagger-ui: to facilitate this work, we added a preprocessing step to convert our swagger schema with our extensions to a valid json schema. It's easier to find a module to represent json schemas than swagger schemas in javascript. By cons we gave up the idea to test the API with the 'try it' button.

It was a year ago, maybe now ...

Refactor your APIs

Many projects don't need anyOf and oneOf, why not us ?

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:
    ...

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”