Fix WithDescendants union type regression in scaffolding

[chillu]

The docs say that you get <type>WithDescendants, but you don't. It's unclear when that happend, but it's a regression which versioned-admin and elemental now rely on. They can't just change their query to use descendant based logic, because that would force them to dynamically create those queries (based on the available types in a particular SilverStripe project).

We could fix this through https://github.com/silverstripe/silverstripe-graphql/issues/209 (RFC: Replace "descendant unions" with fields to simplify querying), but that seems like a long time to leave this regression in the module.

This has also come up with devs on the #graphql channel just now

[chillu]

@unclecheese Just to minimise confusion here, could you validate my assumptions here?

• At some point in the past, <typeWithDescendants> worked, and was documented accordingly
• Then it broke, at least in 3.0.0
• versioned-admin and elemental started relying on this broken behaviour without unions
• We fixed this with https://github.com/silverstripe/silverstripe-graphql/pull/182
• Now versioned-admin and elemental are broken. And the only reason this isn't a widespread problem is because we haven't done a graphql 3.x stable release since https://github.com/silverstripe/silverstripe-graphql/pull/182

[chillu]

Workaround for this in https://github.com/open-sausages/silverstripe-graphql/commit/6dcc005b0178ae689967198dcf27cb5bbbae7d7c

[chillu]

Demo of the current issue in 3.0.1:

graphql.yml

SilverStripe\Control\Director:
  rules:
    'graphql': '%$SilverStripe\GraphQL\Controller.default'
SilverStripe\GraphQL\Manager:
  schemas:
    default:
      scaffolding:
        types:
          Page:
            fields: '*'
            operations:
                read: true
           SilverStripe\ErrorPage\ErrorPage:
             fields: [ErrorCode]

query readPages {
  readPages {
    edges {
      node {
        Title
        ErrorCode
      }
    }
  }
}

Response

{
  "data": null,
  "errors": [
    {
      "message": "Cannot query field \"ErrorCode\" on type \"Page\".",
      "locations": [
        {
          "line": 6,
          "column": 9
        }
      ]
    }
  ]
}

[chillu]

This doesn't work either

graphql.yml

SilverStripe\Control\Director:
  rules:
    'graphql': '%$SilverStripe\GraphQL\Controller.default'
SilverStripe\GraphQL\Manager:
  schemas:
    default:
      scaffolding:
        types:
          Page:
            fields: [Title,ErrorCode]
            operations:
              read: true
          SilverStripe\ErrorPage\ErrorPage:
            fields: [ErrorCode]

Response

{
  "errors": [
    {
      "message": "Invalid field \"ErrorCode\" on Page",
      "code": 0,
      "file": 

[TheBnl]

I was questioning wat the current way of working with descendants is, or if there is an alternative to <type>WithDescendants for version 3.2.3? Those types don't show up in my introspection.

Using the union method as described in the readme section results in the following error: Fragment cannot be spread here as objects of type \"Block\" can never be of type \"VideoBlock\".

I'm asking here because i'm not quite sure what to do, and if this is still a work in progress should the readme be updated stating that it is a WIP feature?