The connection to a to-1 union does not check whether only one node is actually connected.

[AccsoSG]

Describe the bug
With a To-1 relationship to a union, the library does not check whether only one node is actually connected.

Type definitions

type Movie {
  name: String!
  publicationEvent: [PublicationEvent!]! @relationship(type: "HAS_PUBLICATION_EVENT", direction: OUT, nestedOperations: [CONNECT, DISCONNECT])
}

type Episode {
  name: String!
  publicationEvent: [PublicationEvent!]! @relationship(type: "HAS_PUBLICATION_EVENT", direction: OUT, nestedOperations: [CONNECT, DISCONNECT])
}

union MovieOrEpisode = Movie | Episode

type PublicationEvent {
  name: String!
  product: MovieOrEpisode! @relationship(type: "HAS_PUBLICATION_EVENT", direction: IN, nestedOperations: [CONNECT, DISCONNECT])
}

To Reproduce
Steps to reproduce the behavior:

1. Run a server with the following code...
2. Execute the following Mutation...
   (1) Create movies and episodes

mutation CreateMoviesAndEpisodes {
  createMovies(input: [
    {
      name: "TestMovie1"
    },
    {
      name: "TestMovie2"
    }
  ]) {
    movies {
      name
    }
  }

  createEpisodes(input: [
    {
      name: "TestEpisode1"
    },
    {
      name: "TestEpisode2"
    }
  ]){
    episodes {
      name
    }
  }
}

(2) Create PublicationEvent-Object
No filter is specified here for the connection (it has been forgotten). The mutation works anyway. Both movie nodes are connected.

mutation CreatePublicationEvent {
  createPublicationEvents(input: {
    name: "TestPubEvent1",
    product: {
      Movie: {
        connect: {
          where: {
            node: {}
          }
        }
      }
    }
  }){
    publicationEvents {
      name
    }
  }
}

3. Then run the following Query...
   However, only one movie node is returned. It appears undeterministic.

query GetPublicationEvents {
  publicationEvents {
    name
    product {
      ... on Movie {
        name
      }
      ... on Episode {
        name
      }
    }
  }
}

{
  "data": {
    "publicationEvents": [
      {
        "name": "TestPubEvent1",
        "product": {
          "name": "TestMovie2"
        }
      }
    ]
  }
}

Expected behavior
When executing the second mutation, the following error message should be returned: PublicationEvent.product required exactly once

Screenshots

System (please complete the following information):

• OS: [e.g. macOS, Windows]
• Version: @neo4j/graphql@4.4.1
• Node.js version: [e.g. 14.16.0]

Additional context
Add any other context about the problem here.

[neo4j-team-graphql]

Many thanks for raising this bug report @AccsoSG. 🐛 We will now attempt to reproduce the bug based on the steps you have provided.

Please ensure that you've provided the necessary information for a minimal reproduction, including but not limited to:

• Type definitions
• Resolvers
• Query and/or Mutation (or multiple) needed to reproduce

If you have a support agreement with Neo4j, please link this GitHub issue to a new or existing Zendesk ticket.

Thanks again! 🙏

[neo4j-team-graphql]

We've been able to confirm this bug using the steps to reproduce that you provided - many thanks @AccsoSG! 🙏 We will now prioritise the bug and address it appropriately.

[darrellwarde]

Hey @AccsoSG, I'm honestly unsure whether we ever implemented cardinality checks for union relationships, it's something we need to go away and look at. But this could be a feature request as opposed to a bug in that sense. Thanks, happy holidays!