created flatnested tag to convert the anonymous field in a flat map (#46)

* flat fields with the new flatten tag
* added flatten tag documentation

structs.go

@@ -52,6 +52,12 @@ func New(s interface{}) *Struct {
 //   // Map will panic if Animal does not implement String().
 //   Field *Animal `structs:"field,string"`
 //
+// A tag value with the option of "flatten" used in a struct field is to flatten its fields
+// in the output map. Example:
+//
+//   // The FieldStruct's fields will be flattened into the output map.
+//   FieldStruct time.Time `structs:"flatten"`
+//
 // A tag value with the option of "omitnested" stops iterating further if the type
 // is a struct. Example:
 //
@@ -90,7 +96,7 @@ func (s *Struct) FillMap(out map[string]interface{}) {
 	for _, field := range fields {
 		name := field.Name
 		val := s.value.FieldByName(name)
-
+		isSubStruct := false
 		var finalVal interface{}
 
 		tagName, tagOpts := parseTag(field.Tag.Get(s.TagName))
@@ -115,6 +121,7 @@ func (s *Struct) FillMap(out map[string]interface{}) {
 			n := New(val.Interface())
 			n.TagName = s.TagName
 			m := n.Map()
+			isSubStruct = true
 			if len(m) == 0 {
 				finalVal = val.Interface()
 			} else {
@@ -132,9 +139,15 @@ func (s *Struct) FillMap(out map[string]interface{}) {
 			continue
 		}
 
+		if isSubStruct && (tagOpts.Has("flatten")) {
+			for k := range finalVal.(map[string]interface{}) {
+				out[k] = finalVal.(map[string]interface{})[k]
+			}
+		} else {
 			out[name] = finalVal
 		}
 	}
+}
 
 // Values converts the given s struct's field values to a []interface{}.  A
 // struct tag with the content of "-" ignores the that particular field.

structs_test.go

@@ -296,6 +296,60 @@ func TestMap_Anonymous(t *testing.T) {
 	}
 }
 
+func TestMap_Flatnested(t *testing.T) {
+	type A struct {
+		Name string
+	}
+	a := A{Name: "example"}
+
+	type B struct {
+		A `structs:",flatten"`
+		C int
+	}
+	b := &B{C: 123}
+	b.A = a
+
+	m := Map(b)
+
+	_, ok := m["A"].(map[string]interface{})
+	if ok {
+		t.Error("Embedded A struct with tag flatten has to be flat in the map")
+	}
+
+	expectedMap := map[string]interface{}{"Name": "example", "C": 123}
+	if !reflect.DeepEqual(m, expectedMap) {
+		t.Errorf("The exprected map %+v does't correspond to %+v", expectedMap, m)
+	}
+
+}
+
+func TestMap_FlatnestedOverwrite(t *testing.T) {
+	type A struct {
+		Name string
+	}
+	a := A{Name: "example"}
+
+	type B struct {
+		A    `structs:",flatten"`
+		Name string
+		C    int
+	}
+	b := &B{C: 123, Name: "bName"}
+	b.A = a
+
+	m := Map(b)
+
+	_, ok := m["A"].(map[string]interface{})
+	if ok {
+		t.Error("Embedded A struct with tag flatten has to be flat in the map")
+	}
+
+	expectedMap := map[string]interface{}{"Name": "bName", "C": 123}
+	if !reflect.DeepEqual(m, expectedMap) {
+		t.Errorf("The exprected map %+v does't correspond to %+v", expectedMap, m)
+	}
+}
+
 func TestMap_TimeField(t *testing.T) {
 	type A struct {
 		CreatedAt time.Time