gatsby: [gatsby-source-contentful] - "Single Reference" fields w/ multiple types don't generate GraphQL union types

Description

A reference field in Contentful limited to a single Entry but which accepts many types results in an incorrect GraphQL type being generated.

e.g. This “Full Slot field” is a reference to a single Entry, and accepts multiple Entry Types:

However, the fullSlot GraphQL field incorrectly has its type set to ContentfulPlainTextModuleTest instead of a Union(ContentfulPlainTextModuleTest|ContentfulMediaModule) or something similar.

This results in an error running queries when multiple types appear for this reference field. e.g

query AllFullSlices {
  allContentfulSliceFull {
    edges {
      node {
        fullSlot {
          __typename
        }
      }
    }
  }
}

{
  "errors": [
    {
      "message": "Expected value of type \"ContentfulPlainTextModuleTest\" but got: [object Object].",
      "locations": [
        {
          "line": 30,
          "column": 9
        }
      ],
      "path": [
        "allContentfulSliceFull",
        "edges",
        0,
        "node",
        "fullSlot"
      ]
    }
  ],
  "data": {
    "allContentfulSliceFull": {
      "edges": [
        {
          "node": {
            "fullSlot": null
          }
        },
        {
          "node": {
            "fullSlot": {
              "__typename": "ContentfulPlainTextModuleTest"
            }
          }
        }
      ]
    }
  }
}

You can see the inferred type for fullSlot is set incorrectly in Graphiql:

Having done a lot of console.log-ing, it seems like the issue comes from the logic that extracts an “example value” for a given node: https://github.com/gatsbyjs/gatsby/blob/3945d0132d8b3caa32940a2749ab4809bc13d310/packages/gatsby/src/schema/data-tree-utils.js#L52

Maybe a fix is to wrap any Contentful reference fields in an array?

Our current work around is just going to be to convert Contentful references into “many” references so that way the Union types are built correctly and our project can build again.

Environment

Gatsby version: 1.9.145 Node.js version: v8.9.4 Operating System: macOS 10.13.2

Actual result

Gatsby infers a single type for a singular reference field.

Expected behavior

Gatsby should infer a Union type for a singular reference field representing all possible types of the linked Entries.

Steps to reproduce

1. Create a Gatsby content type with a “single reference” field

2. Add two entries to that reference field of different types

3. Observe the generated GraphQL schema for that content type

…

This came out of investigation + context in #3495