Polymorphism in Swagger produces strange error message on discriminator - swagger-2.0

Question

I'm writing a service with a GET that can return one of five different but closely related types. Since the user wants the option of searching through all five types at once, it has to be a single get call. I'm returning JSON, which can easily handle any type.
I'm trying to do this in Swagger, using their polymorphism feature, which I've never tried before. I'm doing it just like in the example, except under "definitions" instead of "components/schemas". But I'm getting a strange error message that I can't understand. The swagger file is below. The error says this:
Schema error at definitions['Event'].discriminator should be string
It gives this on line 49, which says discriminator:
So, my two questions are: How can I fix it? And will this even give me what I need?
swagger: '2.0'
info:
description: RESTful API to retrieve Titles Metadata
version: 1.0.0
title: Swagger Mystery
schemes:
- https
paths:
/event:
get:
operationId: getEvent
summary: searches names
description: |
Search by names, across all types, or by a specific type.
produces:
- application/json
parameters:
- in: query
name: title
description: name to search for
required: true
type: string
- in: query
name: start
required: false
type: boolean
- in: query
name: type
required: false
type: string
description: |
May be "contest", "partner", "sponsor", or "dancer". If missing, will search for all types.
responses:
'200':
description: search results
# I also don't know why I need to comment these out.
# content:
# application/json:
# schema:
# type: array
# items:
# $ref: '#/definitions/Event'
'400':
description: bad input parameter
definitions:
Event:
type: object
discriminator:
propertyName: eventType
properties:
eventType:
type: string
id:
type: integer
format: int64
name:
type: string
description:
type: string
contests:
type: array
items:
$ref: '#/definitions/Contest'
required:
- id
- name
- description
- contests
- eventType
Contest:
allOf:
- $ref: '#/definitions/Event'
- type: object
properties:
parentEvent:
type: string
venue:
type: string
required:
- parentEvent
- venue
Dancer:
allOf:
- $ref: '#/definitions/Event'
- type: object
properties:
eventInvitationDate:
type: string
format: date
venue:
type: string
required:
- eventInvitationDate
- venue
# Sponsor:
# allOf:
# - $ref: '#/definitions/Event'
# - type: object
# properties:
# invitationDate:
# type: string
# format: date
# parentEvent:
# type: string
# partners:
# type: array
# items:
# $ref: '#/definitions/Partner'
Partner:
allOf:
- $ref: '#/definitions/Event'
- type: object
properties:
invitationDate:
type: string
format: date
parentEvent:
type: string
venue:
type: string
required:
- invitationDate
- parentEvent
- venue
# two problems:
# 1. Schema error at definitions['Event'].discriminator
# should be string on line 49 (discriminator:)
# 2. Resolver error:
# e is undefined
# (no line number)
# (This error goes away when I comment out Sponsor.)

Answer 1

The error occurs because you are mixing OpenAPI 2.0 and 3.0 syntax.
Your spec is swagger: '2.0' but the following is 3.0 syntax:
discriminator:
propertyName: eventType
In OpenAPI 2.0, the value of discriminator is the property name:
discriminator: eventType
Also, OpenAPI 2.0 assumes that the possible values of the discriminator property (in this case eventType) are exactly the same as the model names in definitions. That is:
If eventType can be event, partner etc. in lowercase, then the model names must also be in lowercase – event, not Event.
If eventType is some code like e, p, d etc., the model names must be e, p, d etc. instead of Event, Partner etc.
Check out questions for more examples of discriminator usage in OpenAPI 2.0:
Swagger Inheritance and Composition
“discriminator” in polymorphism, OpenAPI 2.0 (Swagger 2.0)
Swagger: variant schema shape dependant on field value