POC: allow users to get requested fields from context

This POC could solve #17

Author: Nick Randall

graphql.go

@@ -140,3 +140,12 @@ func (s *Schema) ToJSON() ([]byte, error) {
 	}
 	return json.MarshalIndent(result, "", "\t")
 }
+
+// GetFieldsFromContext can be used in resolvers to get the fields that were requested in the currenty query for the current type
+func GetFieldsFromContext(ctx context.Context) []string {
+	fields, ok := ctx.Value(exec.ResolverFields).([]string)
+	if !ok {
+		panic("Could not get graphql fields from context")
+	}
+	return fields
+}

internal/exec/exec.go

@@ -25,6 +25,10 @@ const OpenTracingTagTrivial = "graphql.trivial"
 const OpenTracingTagArgsPrefix = "graphql.args."
 const OpenTracingTagError = "graphql.error"
 
+type contextKey string
+
+const ResolverFields contextKey = "Resolver_Fields"
+
 type Exec struct {
 	queryExec    iExec
 	mutationExec iExec
@@ -494,12 +498,14 @@ func (e *objectExec) execSelectionSet(ctx context.Context, r *request, selSet *q
 
 		switch sel := sel.(type) {
 		case *query.Field:
+			namedFields := make(map[string]struct{})
 			if skipByDirective(r, sel.Directives) {
 				continue
 			}
 
+			namedFields = resolveFieldNames("", namedFields, sel.SelSet, r.doc)
 			execSel(func() {
-				e.execField(ctx, r, sel, resolver, addResult)
+				e.execField(context.WithValue(ctx, ResolverFields, namedFields), r, sel, resolver, addResult)
 			})
 
 		case *query.FragmentSpread:
@@ -533,6 +539,32 @@ func (e *objectExec) execSelectionSet(ctx context.Context, r *request, selSet *q
 	wg.Wait()
 }
 
+func resolveFieldNames(prefix string, namedFields map[string]struct{}, selSet *query.SelectionSet, doc *query.Document) map[string]struct{} {
+	if selSet != nil {
+		for _, selectionField := range selSet.Selections {
+			switch selectionField := selectionField.(type) {
+			case *query.Field:
+				if selectionField.SelSet != nil {
+					namedFields = resolveFieldNames(selectionField.Name+".", namedFields, selectionField.SelSet, doc)
+				} else {
+					namedFields[prefix+selectionField.Name] = struct{}{}
+				}
+			case *query.FragmentSpread:
+				frag, ok := doc.Fragments[selectionField.Name]
+				if !ok {
+					panic(fmt.Errorf("fragment %q not found", selectionField.Name)) // TODO proper error handling
+				}
+				namedFields = resolveFieldNames(prefix, namedFields, frag.SelSet, doc)
+			case *query.InlineFragment:
+				namedFields = resolveFieldNames(prefix, namedFields, selectionField.SelSet, doc)
+			default:
+				panic("invalid type")
+			}
+		}
+	}
+	return namedFields
+}
+
 func (e *objectExec) execField(ctx context.Context, r *request, f *query.Field, resolver reflect.Value, addResult addResultFn) {
 	switch f.Name {
 	case "__typename":