Non-mergeable fields error when using UnionTypes

[ryan-cat]

I am running into an issue when using UnionTypes. I receive a non-mergeable fields error when the ObjectTypes being unioned contain the same field names, but the corresponding fields have different types. I am using HotChocolate 10.4.3. I believe this should be valid GraphQL as I have done this with several other frameworks.

Here is a small example:

public class Report {
  public int Id { get; set; }
  public string Title { get; set; }
}

public class ReportType : ObjectType {
  protected override void Configure(IObjectTypeDescriptor descriptor) {
    base.configure(descriptor);
    descriptor.IsOfType((ctx, result) => result.GetType() == typeof(Report));
    descriptor.Field("id").Type<IntType>().Resolver(x => x.Parent<Report>().Id);
    descriptor.Field("title").Type<StringType>().Resolver(x => x.Parent<Report>().Title);
  }
}

public class Finding {
  public int Id { get; set; }
  public string Title { get; set; }
  public string Description { get; set; }
}

public class FindingType : ObjectType {
  protected override void Configure(IObjectTypeDescriptor descriptor) {
    base.configure(descriptor);
    descriptor.IsOfType((ctx, result) => result.GetType() == typeof(Finding));
    descriptor.Field("id").Type<IntType>().Resolver(x => x.Parent<Finding>().Id);
    descriptor.Field("title").Type<NonNullType<StringType>>().Resolver(x => x.Parent<Finding>().Title);
    descriptor.Field("description").Type<StringType>().Resolver(x => x.Parent<Finding>().Description);
  }
}

public class Union : UnionType {
  protected override void Configure(IUnionTypeDescriptor descriptor) {
    base.configure(descriptor);
    descriptor.Type<ReportType>();
    descriptor.Type<FindingType>();
  }
}

Given the schema above, ReportType and FindingType both have an id field with the same type, but a title field with different types. Assuming that the schema above has a root query named "union" that resolves a list of type Union...

This query works fine:

query Query1 {
   union {
     ... on ReportType {
       id
       title
     }
     ... on FindingType {
       id
       description
     }
   }
}

This query does not work fine, receive a non-mergeable fields error because of the mismatching types for the title field:

query Query2 {
   union {
     ... on ReportType {
       id
       title
     }
     ... on FindingType {
       id
       title
       description
     }
   }
}

Thanks in advance for any help. Let me know if you need any more info.

[michaelstaib]

This is as defined in the GraphQL spec:

However, the field responses must be shapes which can be merged. For example, scalar values must not differ. In this example, someValue might be a String or an Int...

http://spec.graphql.org/draft/#sec-Field-Selection-Merging