allow serialization of field having no group tag

sheriff.go

@@ -23,6 +23,10 @@ type Options struct {
 	// Specifying a since setting of "2" with the same API version specified,
 	// will not marshal the field.
 	ApiVersion *version.Version
+	// IncludeEmptyTag determines whether a field without the
+	// `groups` tag should be marshalled ot not.
+	// This option is false by default.
+	IncludeEmptyTag bool
 
 	// This is used internally so that we can propagate anonymous fields groups tag to all child field.
 	nestedGroupsMap map[string][]string
@@ -131,8 +135,21 @@ func Marshal(options *Options, data interface{}) (interface{}, error) {
 				if len(groups) == 0 && options.nestedGroupsMap[field.Name] != nil {
 					groups = append(groups, options.nestedGroupsMap[field.Name]...)
 				}
-				shouldShow := listContains(groups, options.Groups)
-				if !shouldShow || len(groups) == 0 {
+
+				// Marshall the field if
+				// - it has at least one of the requested groups
+				//     or
+				// - it has no group and 'IncludeEmptyTag' is set to true
+				shouldShow := listContains(groups, options.Groups) || (len(groups) == 0 && options.IncludeEmptyTag)
+
+				// Prevent marshalling of the field if
+				// - it should not be shown (above)
+				//     or
+				// - it has no groups and 'IncludeEmptyTag' is set to false
+				shouldHide := !shouldShow || (len(groups) == 0 && !options.IncludeEmptyTag)
+
+				if shouldHide {
+					// skip this field
 					continue
 				}
 			}

sheriff_test.go

@@ -26,6 +26,7 @@ type TestGroupsModel struct {
 	OmitEmptyGroupTest        string            `json:"omit_empty_group_test,omitempty" groups:"test"`
 	SliceString               []string          `json:"slice_string,omitempty" groups:"test"`
 	MapStringStruct           map[string]AModel `json:"map_string_struct,omitempty" groups:"test,test-other"`
+	IncludeEmptyTag           string            `json:"include_empty_tag"`
 }
 
 func TestMarshal_GroupsValidGroup(t *testing.T) {
@@ -141,6 +142,7 @@ func TestMarshal_GroupsNoGroups(t *testing.T) {
 		GroupTestAndOther:  "GroupTestAndOther",
 		OmitEmpty:          "OmitEmpty",
 		OmitEmptyGroupTest: "OmitEmptyGroupTest",
+		IncludeEmptyTag:    "IncludeEmptyTag",
 		MapStringStruct:    map[string]AModel{"firstModel": {true, true}},
 	}
 
@@ -165,6 +167,42 @@ func TestMarshal_GroupsNoGroups(t *testing.T) {
 		},
 		"omit_empty":            "OmitEmpty",
 		"omit_empty_group_test": "OmitEmptyGroupTest",
+		"include_empty_tag":     "IncludeEmptyTag",
+	})
+	assert.NoError(t, err)
+
+	assert.Equal(t, string(expected), string(actual))
+}
+
+func TestMarshal_GroupsValidGroupIncludeEmptyTag(t *testing.T) {
+	testModel := &TestGroupsModel{
+		DefaultMarshal:     "DefaultMarshal",
+		OnlyGroupTest:      "OnlyGroupTest",
+		GroupTestAndOther:  "GroupTestAndOther",
+		OmitEmpty:          "OmitEmpty",
+		OmitEmptyGroupTest: "",
+		SliceString:        []string{"test", "bla"},
+		IncludeEmptyTag:    "IncludeEmptyTag",
+	}
+
+	o := &Options{
+		IncludeEmptyTag: true,
+		Groups:          []string{"test"},
+	}
+
+	actualMap, err := Marshal(o, testModel)
+	assert.NoError(t, err)
+
+	actual, err := json.Marshal(actualMap)
+	assert.NoError(t, err)
+
+	expected, err := json.Marshal(map[string]interface{}{
+		"default_marshal":      "DefaultMarshal",
+		"only_group_test":      "OnlyGroupTest",
+		"group_test_and_other": "GroupTestAndOther",
+		"omit_empty":           "OmitEmpty",
+		"slice_string":         []string{"test", "bla"},
+		"include_empty_tag":    "IncludeEmptyTag",
 	})
 	assert.NoError(t, err)