support to generate model for intermediate interface

_examples/go.mod

@@ -5,7 +5,7 @@ go 1.16
 replace github.com/99designs/gqlgen => ../
 
 require (
-	github.com/99designs/gqlgen v0.0.0-00010101000000-000000000000
+	github.com/99designs/gqlgen v0.16.0
 	github.com/gorilla/websocket v1.4.2
 	github.com/mitchellh/mapstructure v1.4.1
 	github.com/opentracing/opentracing-go v1.2.0

plugin/modelgen/models.go

@@ -36,6 +36,7 @@ type ModelBuild struct {
 type Interface struct {
 	Description string
 	Name        string
+	Implements  []string
 }
 
 type Object struct {
@@ -97,6 +98,7 @@ func (m *Plugin) MutateConfig(cfg *config.Config) error {
 			it := &Interface{
 				Description: schemaType.Description,
 				Name:        schemaType.Name,
+				Implements:  schemaType.Interfaces,
 			}
 
 			b.Interfaces = append(b.Interfaces, it)

plugin/modelgen/models.gotpl

@@ -15,6 +15,9 @@
 {{- range $model := .Interfaces }}
 	{{ with .Description }} {{.|prefixLines "// "}} {{ end }}
 	type {{.Name|go }} interface {
+		{{- range $impl := .Implements }}
+			{{ $impl|go }}
+		{{- end }}
 		Is{{.Name|go }}()
 	}
 {{- end }}

plugin/modelgen/models_test.go

@@ -1,14 +1,17 @@
 package modelgen
 
 import (
+	"go/ast"
 	"go/parser"
 	"go/token"
 	"io/ioutil"
+	"path/filepath"
 	"strings"
 	"testing"
 
 	"github.com/99designs/gqlgen/codegen/config"
 	"github.com/99designs/gqlgen/plugin/modelgen/out"
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
@@ -87,6 +90,93 @@ func TestModelGeneration(t *testing.T) {
 	t.Run("concrete types implement interface", func(t *testing.T) {
 		var _ out.FooBarer = out.FooBarr{}
 	})
+
+	t.Run("implemented interfaces", func(t *testing.T) {
+		pkg, err := parseAst("out")
+		require.NoError(t, err)
+
+		path := filepath.Join("out", "generated.go")
+		generated := pkg.Files[path]
+
+		type field struct {
+			typ  string
+			name string
+		}
+		cases := []struct {
+			name       string
+			wantFields []field
+		}{
+			{
+				name: "A",
+				wantFields: []field{
+					{
+						typ:  "method",
+						name: "IsA",
+					},
+				},
+			},
+			{
+				name: "B",
+				wantFields: []field{
+					{
+						typ:  "method",
+						name: "IsB",
+					},
+				},
+			},
+			{
+				name: "C",
+				wantFields: []field{
+					{
+						typ:  "ident",
+						name: "A",
+					},
+					{
+						typ:  "method",
+						name: "IsC",
+					},
+				},
+			},
+			{
+				name: "D",
+				wantFields: []field{
+					{
+						typ:  "ident",
+						name: "A",
+					},
+					{
+						typ:  "ident",
+						name: "B",
+					},
+					{
+						typ:  "method",
+						name: "IsD",
+					},
+				},
+			},
+		}
+		for _, tc := range cases {
+			tc := tc
+			t.Run(tc.name, func(t *testing.T) {
+				typeSpec, ok := generated.Scope.Lookup(tc.name).Decl.(*ast.TypeSpec)
+				require.True(t, ok)
+
+				fields := typeSpec.Type.(*ast.InterfaceType).Methods.List
+				for i, want := range tc.wantFields {
+					if want.typ == "ident" {
+						ident, ok := fields[i].Type.(*ast.Ident)
+						require.True(t, ok)
+						assert.Equal(t, want.name, ident.Name)
+					}
+					if want.typ == "method" {
+						require.GreaterOrEqual(t, 1, len(fields[i].Names))
+						name := fields[i].Names[0].Name
+						assert.Equal(t, want.name, name)
+					}
+				}
+			})
+		}
+	})
 }
 
 func mutateHook(b *ModelBuild) *ModelBuild {
@@ -98,3 +188,13 @@ func mutateHook(b *ModelBuild) *ModelBuild {
 
 	return b
 }
+
+func parseAst(path string) (*ast.Package, error) {
+	// test setup to parse the types
+	fset := token.NewFileSet()
+	pkgs, err := parser.ParseDir(fset, path, nil, parser.AllErrors)
+	if err != nil {
+		return nil, err
+	}
+	return pkgs["out"], nil
+}

plugin/modelgen/testdata/schema.graphql

@@ -105,3 +105,23 @@ interface Foo_Barer {
 type _Foo_Barr implements  Foo_Barer {
     name: String!
 }
+
+# https://spec.graphql.org/October2021/#sec-Interfaces
+interface A {
+    a: String!
+}
+
+interface B {
+    b: Int!
+}
+
+interface C implements A {
+    a: String!
+    c: Boolean!
+}
+
+interface D implements A & B {
+    a: String!
+    b: Int!
+    d: String
+}