fix(resolvers): use types that implement an interface in __resolveType as first argument

[kamilkisiela]

not interface itself

  /** Mapping between all available schema types and the resolvers types */
  export type ResolversTypes = {
    String: ResolverTypeWrapper<Scalars['String']>,
    Boolean: ResolverTypeWrapper<Scalars['Boolean']>,
-   Book: ResolverTypeWrapper<Book>,
+   Book: ResolversTypes['ColoringBook'] | ResolversTypes['TextBook'],
    TextBook: ResolverTypeWrapper<TextBook>,
    Float: ResolverTypeWrapper<Scalars['Float']>,
    ColoringBook: ResolverTypeWrapper<ColoringBook>,
  };

  /** Mapping between all available schema types and the resolvers parents */
  export type ResolversParentTypes = {
    String: Scalars['String'],
    Boolean: Scalars['Boolean'],
-   Book: Book,
+   Book: ResolversParentTypes['ColoringBook'] | ResolversParentTypes['TextBook'],
    TextBook: TextBook,
    Float: Scalars['Float'],
    ColoringBook: ColoringBook,
  };

  /** Mapping between all available schema types and the resolvers types */
  export type ResolversTypes = {
    String: ResolverTypeWrapper<Scalars['String']>,
    Boolean: ResolverTypeWrapper<Scalars['Boolean']>,
-   Book: ResolverTypeWrapper<Book>,
+   Book: ResolversTypes['ColoringBook'] | ResolversTypes['TextBook'],
    TextBook: ResolverTypeWrapper<TextBook>,
    Float: ResolverTypeWrapper<Scalars['Float']>,
    ColoringBook: ResolverTypeWrapper<ColoringBook>,
  };

  /** Mapping between all available schema types and the resolvers parents */
  export type ResolversParentTypes = {
    String: Scalars['String'],
    Boolean: Scalars['Boolean'],
-   Book: Book,
+   Book: ResolversParentTypes['ColoringBook'] | ResolversParentTypes['TextBook'],
    TextBook: TextBook,
    Float: Scalars['Float'],
    ColoringBook: ColoringBook,
  };

const resolvers: Resolvers = {
  Book: {
    __resolveType(parent, context, info) {
-    // parent is ResolversParentTypes['Book']
+    // parent is ResolversParentTypes['TextBook'] | ResolversParentTypes['ColoringBook']
+
+    // previously `parent.__typename` didn't exist
+    if (parent.__typename === 'TextBook') {
+      return 'TextBook';
+    }
+
+    if (parent.__typename === 'ColoringBook') {
+      return 'ColoringBook';
+    }
+
+    return null;
    },
  },
};

Related #3538

[theguild-bot]

The latest changes of this PR are available as alpha in npm: 1.13.1-alpha-e99e7f6b.5

[zomble]

Great change.

This appears to have a side effect that when an interface doesn't have an implementing type it no longer is omitted in the generated types.

This presents an issue when using https://graphql-modules.com/ and a module defines an interface for others to use, but inside the interface defining module the generated types no longer have that interface.

Module A:
interface Example

Module B:
type Fruit implements Example

Before this version generating types for Module A (in isolation) would include definitions for Example.

After this change, only Module B does as it has an implementing type of Example (which now resolves to Fruit).

@kamilkisiela @dotansimha thoughts? I am aware that from a graph client perspective of a schema, one would not ever care about an interface with out implementing types.

Would it be okay when then are no implementing types to let it generate definitions as it always has done, rather then never?