Add graphql schema aware field level hook to modelgen

docs/content/recipes/modelgen-hook.md

@@ -5,6 +5,8 @@ linkTitle: "Modelgen hook"
 menu: { main: { parent: 'recipes' } }
 ---
 
+## BuildMutateHook
+
 The following recipe shows how to use a `modelgen` plugin hook to mutate generated
 models before they are rendered into a resulting file. This feature has many uses but
 the example focuses only on inserting ORM-specific tags into generated struct fields. This
@@ -77,3 +79,88 @@ type Object struct {
 	field2 *int     `json:"field2" orm_binding:"Object.field2"`
 }
 ```
+
+## FieldMutateHook
+
+For more fine grained control over model generation, a graphql schema aware a FieldHook can be provided. This hook has access to type and field graphql definitions enabling the hook to modify the `modelgen.Field` using directives defined within the schema. 
+
+The below recipe uses this feature to add validate tags to the generated model for use with `go-playground/validator` where the validate tags are defined in a constraint directive in the schema.
+
+``` go
+import (
+	"fmt"
+	"github.com/vektah/gqlparser/v2/ast"
+	"os"
+
+	"github.com/99designs/gqlgen/api"
+	"github.com/99designs/gqlgen/codegen/config"
+	"github.com/99designs/gqlgen/plugin/modelgen"
+)
+
+// Defining mutation function
+func constraintFieldHook(td *ast.Definition, fd *ast.FieldDefinition, f *modelgen.Field) (*modelgen.Field, error) {
+
+	c := fd.Directives.ForName("constraint")
+	if c != nil {
+		formatConstraint := c.Arguments.ForName("format")
+
+		if formatConstraint != nil{
+			f.Tag += " validate:"+formatConstraint.Value.String()
+		}
+
+	}
+
+	return f, nil
+}
+
+func main() {
+	cfg, err := config.LoadConfigFromDefaultLocations()
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "failed to load config", err.Error())
+		os.Exit(2)
+	}
+
+	// Attaching the mutation function onto modelgen plugin
+	p := modelgen.Plugin{
+		FieldHook: constraintFieldHook,
+	}
+
+	err = api.Generate(cfg,
+		api.NoPlugins(),
+		api.AddPlugin(&p),
+	)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err.Error())
+		os.Exit(3)
+	}
+}
+```
+
+This schema:
+
+```graphql
+directive @constraint(
+    format: String
+) on INPUT_FIELD_DEFINITION | FIELD_DEFINITION
+
+input ObjectInput {
+    contactEmail: String @constraint(format: "email")
+    website: String @constraint(format: "uri")
+}
+```
+
+Will generate the model:
+
+```go
+type ObjectInput struct {
+	contactEmail *string  `json:"contactEmail" validate:"email"`
+	website      *string  `json:"website" validate:"uri"`
+}
+```
+
+If a constraint being used during generation shoud not be published during introspection, the directive should be listed with `skip_runtime:true` in gqlgen.yml
+```yaml
+directives:
+  constraint:
+    skip_runtime: true
+```

plugin/modelgen/models.go

@@ -13,6 +13,11 @@ import (
 
 type BuildMutateHook = func(b *ModelBuild) *ModelBuild
 
+type FieldMutateHook = func(td *ast.Definition, fd *ast.FieldDefinition, f *Field) (*Field, error)
+
+func defaultFieldMutateHook(td *ast.Definition, fd *ast.FieldDefinition, f *Field) (*Field, error) {
+	return f, nil
+}
 func defaultBuildMutateHook(b *ModelBuild) *ModelBuild {
 	return b
 }
@@ -58,11 +63,13 @@ type EnumValue struct {
 func New() plugin.Plugin {
 	return &Plugin{
 		MutateHook: defaultBuildMutateHook,
+		FieldHook:  defaultFieldMutateHook,
 	}
 }
 
 type Plugin struct {
 	MutateHook BuildMutateHook
+	FieldHook  FieldMutateHook
 }
 
 var _ plugin.ConfigMutator = &Plugin{}
@@ -167,12 +174,22 @@ func (m *Plugin) MutateConfig(cfg *config.Config) error {
 					tag = tag + " " + extraTag
 				}
 
-				it.Fields = append(it.Fields, &Field{
+				f := &Field{
 					Name:        name,
 					Type:        typ,
 					Description: field.Description,
 					Tag:         tag,
-				})
+				}
+
+				if m.FieldHook != nil {
+					mf, err := m.FieldHook(schemaType, field, f)
+					if err != nil {
+						return fmt.Errorf("generror: field %v.%v: %w", it.Name, field.Name, err)
+					}
+					f = mf
+				}
+
+				it.Fields = append(it.Fields, f)
 			}
 
 			b.Models = append(b.Models, it)

plugin/modelgen/models_test.go

@@ -7,6 +7,8 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/vektah/gqlparser/v2/ast"
+
 	"github.com/99designs/gqlgen/codegen/config"
 	"github.com/99designs/gqlgen/plugin/modelgen/out"
 	"github.com/stretchr/testify/require"
@@ -18,6 +20,7 @@ func TestModelGeneration(t *testing.T) {
 	require.NoError(t, cfg.Init())
 	p := Plugin{
 		MutateHook: mutateHook,
+		FieldHook:  mutateFieldHook,
 	}
 	require.NoError(t, p.MutateConfig(cfg))
 
@@ -65,6 +68,22 @@ func TestModelGeneration(t *testing.T) {
 		}
 	})
 
+	t.Run("field hooks are applied", func(t *testing.T) {
+		file, err := ioutil.ReadFile("./out/generated.go")
+		require.NoError(t, err)
+
+		fileText := string(file)
+
+		expectedTags := []string{
+			`json:"name" anotherTag:"tag"`,
+			`json:"enum" yetAnotherTag:"12"`,
+		}
+
+		for _, tag := range expectedTags {
+			require.True(t, strings.Contains(fileText, tag))
+		}
+	})
+
 	t.Run("concrete types implement interface", func(t *testing.T) {
 		var _ out.FooBarer = out.FooBarr{}
 	})
@@ -79,3 +98,19 @@ func mutateHook(b *ModelBuild) *ModelBuild {
 
 	return b
 }
+
+func mutateFieldHook(td *ast.Definition, fd *ast.FieldDefinition, f *Field) (*Field, error) {
+
+	if fd.Directives == nil || td.Name != "FieldMutationHook" {
+		return f, nil
+	}
+
+	directive := fd.Directives.ForName("addTag")
+	if directive != nil {
+		args := directive.ArgumentMap(map[string]interface{}{})
+		if tag, ok := args["tag"]; ok {
+			f.Tag += " " + tag.(string)
+		}
+	}
+	return f, nil
+}

plugin/modelgen/testdata/schema.graphql

@@ -1,3 +1,6 @@
+directive @addTag(tag: String!) on INPUT_FIELD_DEFINITION
+    | FIELD_DEFINITION
+
 type Query {
     thisShoudlntGetGenerated: Boolean
 }
@@ -54,6 +57,11 @@ input ExistingInput {
     enum: ExistingEnum
 }
 
+type FieldMutationHook {
+    name: String @addTag(tag :"anotherTag:\"tag\"")
+    enum: ExistingEnum @addTag(tag: "yetAnotherTag:\"12\"")
+}
+
 enum ExistingEnum {
     Hello
     Goodbye