feat(transforms): map object fields to new structures

Provides building blocks to allow transforming object types by wrapping a field with a new object type or by extracting subfields from a field of a certain object type into the parent:

 = Adds new ExtendSchema transformer to extend the wrapping schema within transformSchema with new fields.
 = Adds new functions that create resolvers that can wrap, extract, and rename merged fields, for use within the ExtendSchema transformer
 = Adds new MapFields transformer to allow transformation of field nodes within a request by object type and field name.

src/Interfaces.ts

@@ -30,6 +30,7 @@ export interface IResolverValidationOptions {
 export interface IAddResolveFunctionsToSchemaOptions {
   schema: GraphQLSchema;
   resolvers: IResolvers;
+  defaultFieldResolver?: IFieldResolver<any, any>;
   resolverValidationOptions?: IResolverValidationOptions;
   inheritResolversFromInterfaces?: boolean;
 }

src/generate/addResolveFunctionsToSchema.ts

@@ -18,7 +18,7 @@ import {
 } from '../Interfaces';
 import { applySchemaTransforms } from '../transforms/transforms';
 import { checkForResolveTypeResolver, extendResolversFromInterfaces } from '.';
-import AddEnumAndScalarResolvers from '../transforms/AddEnumAndScalarResolvers';
+import { AddDefaultResolver, AddEnumAndScalarResolvers } from '../transforms/index';
 
 function addResolveFunctionsToSchema(
   options: IAddResolveFunctionsToSchemaOptions | GraphQLSchema,
@@ -39,6 +39,7 @@ function addResolveFunctionsToSchema(
   const {
     schema,
     resolvers: inputResolvers,
+    defaultFieldResolver,
     resolverValidationOptions = {},
     inheritResolversFromInterfaces = false,
   } = options;
@@ -153,12 +154,13 @@ function addResolveFunctionsToSchema(
 
   checkForResolveTypeResolver(schema, requireResolversForResolveType);
 
-  // If there are any enum resolver functions (that are used to return
-  // internal enum values), create a new schema that includes enums with the
-  // new internal facing values.
-  // also parse all defaultValues in all input fields to use internal values for enums/scalars
   const updatedSchema = applySchemaTransforms(schema, [
+    // If there are any enum resolver functions (that are used to return
+    // internal enum values), create a new schema that includes enums with the
+    // new internal facing values.
+    // also parse all defaultValues in all input fields to use internal values for enums/scalars
     new AddEnumAndScalarResolvers(enumValueMap, scalarTypeMap),
+    new AddDefaultResolver(defaultFieldResolver),
   ]);
 
   return updatedSchema;

src/stitching/defaultMergedResolver.ts

@@ -1,4 +1,4 @@
-import { GraphQLFieldResolver } from 'graphql';
+import { GraphQLFieldResolver, defaultFieldResolver } from 'graphql';
 import { getErrorsFromParent } from './errors';
 import { handleResult } from './checkResultAndHandleErrors';
 import { getResponseKeyFromInfo } from './getResponseKeyFromInfo';
@@ -18,10 +18,7 @@ const defaultMergedResolver: GraphQLFieldResolver<any, any> = (parent, args, con
   // check to see if parent is not a proxied result, i.e. if parent resolver was manually overwritten
   // See https://github.com/apollographql/graphql-tools/issues/967
   if (!Array.isArray(errors)) {
-    if (typeof parent[info.fieldName] === 'function') {
-      return parent[info.fieldName](parent, args, context, info);
-    }
-    return parent[info.fieldName];
+    return defaultFieldResolver(parent, args, context, info);
   }
 
   return handleResult(info, parent[responseKey], errors);