Add modelgen hook recipe

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"`
+}
+```