allow selectionSet hints for stitching to be functions

packages/delegate/src/delegationBindings.ts

@@ -40,9 +40,10 @@ export function defaultDelegationBinding(delegationContext: DelegationContext):
     delegationTransforms = delegationTransforms.concat([
       new AddSelectionSets(
         info.schema,
+        returnType,
         stitchingInfo.selectionSetsByType,
         stitchingInfo.selectionSetsByField,
-        returnType
+        stitchingInfo.dynamicSelectionSetsByField
       ),
       new WrapConcreteTypes(returnType, transformedSchema),
       new ExpandAbstractTypes(info.schema, transformedSchema),

packages/delegate/src/transforms/AddSelectionSets.ts

@@ -1,4 +1,4 @@
-import { GraphQLSchema, SelectionSetNode, TypeInfo, GraphQLOutputType, Kind } from 'graphql';
+import { GraphQLSchema, SelectionSetNode, TypeInfo, GraphQLOutputType, Kind, FieldNode } from 'graphql';
 
 import { Transform, Request } from '@graphql-tools/utils';
 import VisitSelectionSets from './VisitSelectionSets';
@@ -8,12 +8,13 @@ export default class AddSelectionSetsByField implements Transform {
 
   constructor(
     sourceSchema: GraphQLSchema,
+    initialType: GraphQLOutputType,
     selectionSetsByType: Record<string, SelectionSetNode>,
     selectionSetsByField: Record<string, Record<string, SelectionSetNode>>,
-    initialType: GraphQLOutputType
+    dynamicSelectionSetsByField: Record<string, Record<string, Array<(node: FieldNode) => SelectionSetNode>>>
   ) {
     this.transformer = new VisitSelectionSets(sourceSchema, initialType, (node, typeInfo) =>
-      visitSelectionSet(node, typeInfo, selectionSetsByType, selectionSetsByField)
+      visitSelectionSet(node, typeInfo, selectionSetsByType, selectionSetsByField, dynamicSelectionSetsByField)
     );
   }
 
@@ -26,7 +27,8 @@ function visitSelectionSet(
   node: SelectionSetNode,
   typeInfo: TypeInfo,
   selectionSetsByType: Record<string, SelectionSetNode>,
-  selectionSetsByField: Record<string, Record<string, SelectionSetNode>>
+  selectionSetsByField: Record<string, Record<string, SelectionSetNode>>,
+  dynamicSelectionSetsByField: Record<string, Record<string, Array<(node: FieldNode) => SelectionSetNode>>>
 ): SelectionSetNode {
   const parentType = typeInfo.getParentType();
   if (parentType != null) {
@@ -52,6 +54,23 @@ function visitSelectionSet(
       });
     }
 
+    if (parentTypeName in dynamicSelectionSetsByField) {
+      node.selections.forEach(selection => {
+        if (selection.kind === Kind.FIELD) {
+          const name = selection.name.value;
+          const dynamicSelectionSets = dynamicSelectionSetsByField[parentTypeName][name];
+          if (dynamicSelectionSets != null) {
+            dynamicSelectionSets.forEach(selectionSetFn => {
+              const selectionSet = selectionSetFn(selection);
+              if (selectionSet != null) {
+                selections = selections.concat(selectionSet.selections);
+              }
+            });
+          }
+        }
+      });
+    }
+
     if (selections !== node.selections) {
       return {
         ...node,

packages/delegate/src/types.ts

@@ -151,7 +151,8 @@ export type MergedTypeResolver = (
 export interface StitchingInfo {
   transformedSchemas: Map<GraphQLSchema | SubschemaConfig, GraphQLSchema>;
   fragmentsByField: Record<string, Record<string, InlineFragmentNode>>;
-  selectionSetsByField: Record<string, Record<string, SelectionSetNode>>;
   selectionSetsByType: Record<string, SelectionSetNode>;
+  selectionSetsByField: Record<string, Record<string, SelectionSetNode>>;
+  dynamicSelectionSetsByField: Record<string, Record<string, Array<(node: FieldNode) => SelectionSetNode>>>;
   mergedTypes: Record<string, MergedTypeInfo>;
 }

packages/stitch/src/stitchingInfo.ts

@@ -47,8 +47,9 @@ export function createStitchingInfo(
   return {
     transformedSchemas,
     fragmentsByField: undefined,
-    selectionSetsByField: undefined,
     selectionSetsByType,
+    selectionSetsByField: undefined,
+    dynamicSelectionSetsByField: undefined,
     mergedTypes,
   };
 }
@@ -194,6 +195,7 @@ function createMergedTypes(
 
 export function completeStitchingInfo(stitchingInfo: StitchingInfo, resolvers: IResolvers): StitchingInfo {
   const selectionSetsByField = Object.create(null);
+  const dynamicSelectionSetsByField = Object.create(null);
   const rawFragments: Array<{ field: string; fragment: string }> = [];
 
   Object.keys(resolvers).forEach(typeName => {
@@ -204,20 +206,32 @@ export function completeStitchingInfo(stitchingInfo: StitchingInfo, resolvers: I
     Object.keys(type).forEach(fieldName => {
       const field = type[fieldName] as IFieldResolverOptions;
       if (field.selectionSet) {
-        const selectionSet = parseSelectionSet(field.selectionSet);
-        if (!(typeName in selectionSetsByField)) {
-          selectionSetsByField[typeName] = Object.create(null);
-        }
+        if (typeof field.selectionSet === 'function') {
+          if (!(typeName in selectionSetsByField)) {
+            dynamicSelectionSetsByField[typeName] = Object.create(null);
+          }
 
-        if (!(fieldName in selectionSetsByField[typeName])) {
-          selectionSetsByField[typeName][fieldName] = {
-            kind: Kind.SELECTION_SET,
-            selections: [],
-          };
+          if (!(fieldName in selectionSetsByField[typeName])) {
+            dynamicSelectionSetsByField[typeName][fieldName] = [];
+          }
+
+          dynamicSelectionSetsByField[typeName][fieldName].push(field.selectionSet);
+        } else {
+          const selectionSet = parseSelectionSet(field.selectionSet);
+          if (!(typeName in selectionSetsByField)) {
+            selectionSetsByField[typeName] = Object.create(null);
+          }
+
+          if (!(fieldName in selectionSetsByField[typeName])) {
+            selectionSetsByField[typeName][fieldName] = {
+              kind: Kind.SELECTION_SET,
+              selections: [],
+            };
+          }
+          selectionSetsByField[typeName][fieldName].selections = selectionSetsByField[typeName][
+            fieldName
+          ].selections.concat(selectionSet.selections);
         }
-        selectionSetsByField[typeName][fieldName].selections = selectionSetsByField[typeName][
-          fieldName
-        ].selections.concat(selectionSet.selections);
       }
       if (field.fragment) {
         rawFragments.push({
@@ -254,6 +268,7 @@ export function completeStitchingInfo(stitchingInfo: StitchingInfo, resolvers: I
   });
 
   stitchingInfo.selectionSetsByField = selectionSetsByField;
+  stitchingInfo.dynamicSelectionSetsByField = dynamicSelectionSetsByField;
   stitchingInfo.fragmentsByField = fragmentsByField;
 
   return stitchingInfo;

packages/stitch/src/types.ts

@@ -5,6 +5,7 @@ import {
   DocumentNode,
   SelectionNode,
   InlineFragmentNode,
+  FieldNode,
 } from 'graphql';
 import { ITypeDefinitions, TypeMap } from '@graphql-tools/utils';
 import { SubschemaConfig } from '@graphql-tools/delegate';
@@ -32,8 +33,9 @@ export interface MergedTypeInfo {
 export interface StitchingInfo {
   transformedSchemas: Map<GraphQLSchema | SubschemaConfig, GraphQLSchema>;
   fragmentsByField: Record<string, Record<string, InlineFragmentNode>>;
-  selectionSetsByField: Record<string, Record<string, SelectionSetNode>>;
   selectionSetsByType: Record<string, SelectionSetNode>;
+  selectionSetsByField: Record<string, Record<string, SelectionSetNode>>;
+  dynamicSelectionSetsByField: Record<string, Record<string, Array<(node: FieldNode) => SelectionSetNode>>>;
   mergedTypes: Record<string, MergedTypeInfo>;
 }
 
@@ -65,6 +67,6 @@ export type OnTypeConflict = (
 declare module '@graphql-tools/utils' {
   interface IFieldResolverOptions<TSource = any, TContext = any, TArgs = any> {
     fragment?: string;
-    selectionSet?: string;
+    selectionSet?: string | ((node: FieldNode) => SelectionSetNode);
   }
 }