FilterTypes Transform can break an UnionType

[AndKiel]

Given:

• ObjectTypes A, B, and C
• UnionType with types A, B, and C

When:

• FilterType transform is used to hide one of the ObjectTypes included in the UnionType

Then:

• ObjectType which was hidden becomes null inside UnionType, making schema invalid

My current workaround is removing null members and recreating UnionTypes without them:

import { GraphQLNamedType, GraphQLSchema, GraphQLUnionType } from "graphql";
import { Transform } from "graphql-tools-fork";
import { visitSchema, VisitSchemaKind } from "graphql-tools-fork/dist/transforms/visitSchema";

export class CleanUpUnionTypes implements Transform {
  public transformSchema(originalSchema: GraphQLSchema): GraphQLSchema {
    return visitSchema(originalSchema, {
      [VisitSchemaKind.UNION_TYPE](type): GraphQLNamedType {
        const config = (<GraphQLUnionType>type).toConfig();
        config.types = config.types.filter(Boolean);
        return new GraphQLUnionType(config);
      }
    });
  }
}

The question is, should this be responsibility of FilterType transform to clean up unions as an additional step?

[yaacovCR]

I think so! My hastily put together filterSchema function should do this, you can possibly try that instead of the FilterTypes transform.

[AndKiel]

There is one more edge-case related to types filtering.
When you filter a type and another type has a field using that type, and it has no other fields, it becomes an "empty" type, also making schema invalid.

EDIT: (that can be opened as another issue, I guess, would be blocked by #14 because it's impossible to write custom transform returning null/undefined at the moment)

[yaacovCR]

Should be fixed in v6.4 thanks to the use of healing/pruning within the new consolidated visitSchema. Have to work on docs.