Add recipe for using the modelgen FieldMutateHook

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
+```