Fix marshaling interfaces and union types

codegen/interface.gotpl

@@ -6,6 +6,9 @@ func (ec *executionContext) _{{$interface.Name}}(ctx context.Context, sel ast.Se
 		return graphql.Null
 	{{- range $implementor := $interface.Implementors }}
 		case {{$implementor.Type | ref}}:
+			if len(graphql.CollectFields(ec.OperationContext, sel, []string{"{{$interface.Type | typeName }}", "{{$implementor.Type | typeName}}"})) == 0 {
+				return graphql.Empty{}
+			}
 			{{- if $implementor.CanBeNil }}
 				if obj == nil {
 					return graphql.Null

codegen/templates/templates.go

@@ -88,15 +88,15 @@ func Render(cfg Options) error {
 	}
 
 	roots := make([]string, 0, len(t.Templates()))
-	for _, template := range t.Templates() {
+	for _, templ := range t.Templates() {
 		// templates that end with _.gotpl are special files we don't want to include
-		if strings.HasSuffix(template.Name(), "_.gotpl") ||
+		if strings.HasSuffix(templ.Name(), "_.gotpl") ||
 			// filter out templates added with {{ template xxx }} syntax inside the template file
-			!strings.HasSuffix(template.Name(), ".gotpl") {
+			!strings.HasSuffix(templ.Name(), ".gotpl") {
 			continue
 		}
 
-		roots = append(roots, template.Name())
+		roots = append(roots, templ.Name())
 	}
 
 	// then execute all the important looking ones in order, adding them to the same file
@@ -220,6 +220,7 @@ func Funcs() template.FuncMap {
 		"render": func(filename string, tpldata any) (*bytes.Buffer, error) {
 			return render(resolveName(filename, 0), tpldata)
 		},
+		"typeName": typeName,
 	}
 }
 
@@ -647,6 +648,16 @@ func resolveName(name string, skip int) string {
 	return filepath.Join(filepath.Dir(callerFile), name)
 }
 
+func typeName(t types.Type) string {
+	name := types.TypeString(t, func(*types.Package) string {
+		return ""
+	})
+	if name != "" && strings.HasPrefix(name, "*") {
+		return name[1:]
+	}
+	return name
+}
+
 func render(filename string, tpldata any) (*bytes.Buffer, error) {
 	t := template.New("").Funcs(Funcs())
 
@@ -672,7 +683,7 @@ func write(filename string, b []byte, packages *code.Packages) error {
 
 	formatted, err := imports.Prune(filename, b, packages)
 	if err != nil {
-		fmt.Fprintf(os.Stderr, "gofmt failed on %s: %s\n", filepath.Base(filename), err.Error())
+		_, _ = fmt.Fprintf(os.Stderr, "gofmt failed on %s: %s\n", filepath.Base(filename), err.Error())
 		formatted = b
 	}
 

codegen/templates/templates_test.go

@@ -3,6 +3,7 @@ package templates
 import (
 	"embed"
 	"fmt"
+	"go/types"
 	"os"
 	"path/filepath"
 	"testing"
@@ -355,3 +356,28 @@ func TestRenderFS(t *testing.T) {
 	// don't look at last character since it's \n on Linux and \r\n on Windows
 	assert.Equal(t, expectedString, actualContentsStr[:len(expectedString)])
 }
+
+func TestTypeName(t *testing.T) {
+	testType := types.NewNamed(
+		types.NewTypeName(0, types.NewPackage(
+			"github.com/99designs/gqlgen/codegen/templates",
+			"templates",
+		), "testType", nil),
+		types.NewStruct(nil, nil),
+		nil,
+	)
+
+	tests := []struct {
+		input    types.Type
+		expected string
+	}{
+		{testType, "testType"},
+		{types.NewPointer(testType), "testType"},
+		{types.NewPointer(types.NewPointer(testType)), "*testType"},
+	}
+
+	for _, test := range tests {
+		result := typeName(test.input)
+		assert.Equal(t, test.expected, result)
+	}
+}

codegen/testserver/followschema/generated_test.go

@@ -37,6 +37,12 @@ func TestUnionFragments(t *testing.T) {
 	resolvers.QueryResolver.ShapeUnion = func(ctx context.Context) (ShapeUnion, error) {
 		return &Circle{Radius: 32}, nil
 	}
+	resolvers.QueryResolver.Shapes = func(ctx context.Context) ([]Shape, error) {
+		return []Shape{
+			&Circle{Radius: 45},
+			&Circle{Radius: 54},
+		}, nil
+	}
 
 	srv := handler.NewDefaultServer(NewExecutableSchema(Config{Resolvers: resolvers}))
 	c := client.New(srv)
@@ -78,4 +84,23 @@ func TestUnionFragments(t *testing.T) {
 		`, &resp)
 		require.NotEmpty(t, resp.ShapeUnion.Radius)
 	})
+
+	t.Run("without circle", func(t *testing.T) {
+		var resp struct {
+			Shapes []struct {
+				Length, Width float64
+			}
+		}
+		require.Empty(t, resp.Shapes)
+		c.MustPost(`query {
+			shapes {
+				... on Rectangle {
+					length
+					width
+				}
+			}
+		}
+		`, &resp)
+		require.Empty(t, resp.Shapes)
+	})
 }