Server cannot unmarshal array of polymorphic values

[ntoljic]

Problem statement

Not sure if this is related to #842, but when an endpoint expects an array of polymorphic values, the server fails with the following error message:

{"code":400,"message":"parsing contents body from \"\" failed, because json: cannot unmarshal object into Go value of type models.Value"}

Swagger specification

swagger: '2.0'
info:
  version: '0.0.1'
  title: Test
schemes:
  - http
basePath: /v1
produces:
  - application/json
consumes:
  - application/json
paths:
  /test:
    put:
      operationId: test
      parameters:
      - name: payload
        in: body
        required: true
        schema:
          type: array
          items:
            $ref: '#/definitions/Value'
      tags:
        - Test
      responses:
        default:
          description: "successful operation"
definitions:
  Value:
    type: object
    discriminator: ValueType
    required:
    - ValueType
    properties:
      ValueType:
        type: string
  StringValue:
    allOf:
    - $ref: '#/definitions/Value'
    - type: object
      properties:
        foobar:
          type: string

Steps to reproduce

• Implement a dummy handler
• Build and start the server
• execute the following curl:

curl 'http://localhost:PORT/v1/test -X PUT -H 'Content-type: application/json' -d '[{"ValueType":"StringValue","Foobar":"hello"}]'

Environment

swagger version: 0.12.0
go version: go1.9.1 darwin/amd64
OS: MacOS but should be irrelevant

Workaround

When payload is to be an object that contains the array instead of just the array, it works.

- name: payload
    in: body
    required: true
    schema:
      type: object
      properties:
        values:
          type: array
          items:
            $ref: '#/definitions/Value'

Since the issue can be worked around it is not a blocking issue for me, I am submitting this issue more out of curiosity as to what provokes this behavior.

Edit: fix invalid sample spec

[gregmarr]

You're using a really old version. Please try with the latest release, this should be fixed.

[ntoljic]

Just upgraded to 0.15.0.

Same result :/

[gregmarr]

I guess it's different because of this: When payload is to be an object that contains the array instead of just the array, it works.

[fredbi]

Could you please add here the BindRequest() method generated for your operation?

[ntoljic]

Sure thing. Here you go:

// BindRequest both binds and validates a request, it assumes that complex things implement a Validatable(strfmt.Registry) error interface
// for simple values it will use straight method calls.
//
// To ensure default values, the struct must have been initialized with NewTestParams() beforehand.
func (o *TestParams) BindRequest(r *http.Request, route *middleware.MatchedRoute) error {
	var res []error

	o.HTTPRequest = r

	if runtime.HasBody(r) {
		defer r.Body.Close()
		var body []models.Value
		if err := route.Consumer.Consume(r.Body, &body); err != nil {
			if err == io.EOF {
				res = append(res, errors.Required("payload", "body"))
			} else {
				res = append(res, errors.NewParseError("payload", "body", "", err))
			}
		} else {
			// validate array of body objects
			for i := range body {
				if err := body[i].Validate(route.Formats); err != nil {
					res = append(res, err)
					break
				}
			}

			if len(res) == 0 {
				o.Payload = body
			}
		}
	} else {
		res = append(res, errors.Required("payload", "body"))
	}
	if len(res) > 0 {
		return errors.CompositeValidationError(res...)
	}
	return nil
}

[fredbi]

Yes I've seen already this when trying to fix some other stuff with polymorphism.
Problem is that the array type does not get its "IsExported" flag set.

[fredbi]

I did not release this fix, because there are many other problems here.
I may fix this, but then how about [][]Value, or map[string]Value, no even thinking of [][]map[string][][]Value... They all fail. So that is a bit more complicated.

[ntoljic]

Well, if this cannot be fixed for all possible cases it should at least be documented as a limitation of go-swagger, don't you think? Not sure, how heavily used this feature is anyway.
Would be nice to have it working for the 'simple' case though ;)

[fredbi]

i am not saying this cannot be fixed. I am saying it is not fixed yet because it is not a simplistic issue.
About documenting limitations, many things have been added recently. Did you read it?

[ntoljic]

You got me there. I definitely have to catch up on the docs.
In the meantime, I can work around this issue by wrapping the array in an object.
Happy to test any fixes, although you seem to have a much better understanding of the wider implications and possible edge cases.

[fredbi]

Found minimal patch to make this one work.

There are many other edge cases with polymorphism.
ad this story shall also be told.