Add possibility to hook into modelgen plugin

docs/content/recipes/modelgen-hook.md

@@ -0,0 +1,79 @@
+---
+title: "Allowing mutation of generated models before rendering"
+description: How to use a model mutation function to insert a ORM-specific tags onto struct fields.
+linkTitle: "Modelgen hook"
+menu: { main: { parent: 'recipes' } }
+---
+
+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
+is a common use case since it allows for better field matching of DB queries and
+the generated data structure.
+
+First of all, we need to create a function that will mutate the generated model.
+Then we can attach the function to the plugin and use it like any other plugin.
+
+``` go
+import (
+	"fmt"
+	"os"
+
+	"github.com/99designs/gqlgen/api"
+	"github.com/99designs/gqlgen/codegen/config"
+	"github.com/99designs/gqlgen/plugin/modelgen"
+)
+
+// Defining mutation function
+func mutateHook(b *ModelBuild) *ModelBuild {
+	for _, model := range b.Models {
+		for _, field := range model.Fields {
+			field.Tag += ` orm_binding:"` + model.Name + `.` +  field.Name + `"`
+		}
+	}
+
+	return b
+}
+
+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{
+		MutateHook: mutateHook,
+	}
+
+	err = api.Generate(cfg,
+		api.NoPlugins(),
+		api.AddPlugin(p),
+	)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err.Error())
+		os.Exit(3)
+	}
+}
+```
+
+Now fields from generated models will contain a additional tag `orm_binding`.
+
+This schema:
+
+```graphql
+type Object {
+    field1: String
+    field2: Int
+}
+```
+
+Will gen generated into:
+
+```go
+type Object struct {
+	field1 *string  `json:"field1" orm_binding:"Object.field1"`
+	field2 *int     `json:"field2" orm_binding:"Object.field2"`
+}
+```

plugin/modelgen/models.go

@@ -11,6 +11,12 @@ import (
 	"github.com/vektah/gqlparser/ast"
 )
 
+type BuildMutateHook = func(b *ModelBuild) *ModelBuild
+
+func defaultBuildMutateHook(b *ModelBuild) *ModelBuild {
+	return b
+}
+
 type ModelBuild struct {
 	PackageName string
 	Interfaces  []*Interface
@@ -50,10 +56,14 @@ type EnumValue struct {
 }
 
 func New() plugin.Plugin {
-	return &Plugin{}
+	return &Plugin{
+		MutateHook: defaultBuildMutateHook,
+	}
 }
 
-type Plugin struct{}
+type Plugin struct {
+	MutateHook BuildMutateHook
+}
 
 var _ plugin.ConfigMutator = &Plugin{}
 
@@ -221,6 +231,10 @@ func (m *Plugin) MutateConfig(cfg *config.Config) error {
 		return nil
 	}
 
+	if m.MutateHook != nil {
+		b = m.MutateHook(b)
+	}
+
 	return templates.Render(templates.Options{
 		PackageName:     cfg.Model.Package,
 		Filename:        cfg.Model.Filename,

plugin/modelgen/models_test.go

@@ -14,7 +14,9 @@ import (
 func TestModelGeneration(t *testing.T) {
 	cfg, err := config.LoadConfig("testdata/gqlgen.yml")
 	require.NoError(t, err)
-	p := Plugin{}
+	p := Plugin{
+		MutateHook: mutateHook,
+	}
 	require.NoError(t, p.MutateConfig(cfg))
 
 	require.True(t, cfg.Models.UserDefined("MissingTypeNotNull"))
@@ -42,4 +44,32 @@ func TestModelGeneration(t *testing.T) {
 			require.True(t, len(words) > 1, "expected description %q to have more than one word", text)
 		}
 	})
+
+	t.Run("tags are applied", func(t *testing.T) {
+		file, err := ioutil.ReadFile("./out/generated.go")
+		require.NoError(t, err)
+
+		fileText := string(file)
+
+		expectedTags := []string{
+			`json:"missing2" database:"MissingTypeNotNullmissing2"`,
+			`json:"name" database:"MissingInputname"`,
+			`json:"missing2" database:"MissingTypeNullablemissing2"`,
+			`json:"name" database:"TypeWithDescriptionname"`,
+		}
+
+		for _, tag := range expectedTags {
+			require.True(t, strings.Contains(fileText, tag))
+		}
+	})
+}
+
+func mutateHook(b *ModelBuild) *ModelBuild {
+	for _, model := range b.Models {
+		for _, field := range model.Fields {
+			field.Tag += ` database:"` + model.Name + field.Name + `"`
+		}
+	}
+
+	return b
 }