Fallback to JSON tags when BSON tag isn't present

bson/bson.go

@@ -698,9 +698,21 @@ func getStructInfo(st reflect.Type) (*structInfo, error) {
 		info := fieldInfo{Num: i}
 
 		tag := field.Tag.Get("bson")
-		if tag == "" && strings.Index(string(field.Tag), ":") < 0 {
-			tag = string(field.Tag)
+
+		// Fall-back to JSON struct tag, if feature flag is set.
+		if tag == "" && useJSONTagFallback {
+			tag = field.Tag.Get("json")
 		}
+
+		// If there's no bson/json tag available.
+		if tag == "" {
+			// If there's no tag, and also no tag: value splits (i.e. no colon)
+			// then assume the entire tag is the value
+			if strings.Index(string(field.Tag), ":") < 0 {
+				tag = string(field.Tag)
+			}
+		}
+
 		if tag == "-" {
 			continue
 		}

bson/compatability_test.go

@@ -0,0 +1,54 @@
+package bson_test
+
+import (
+	"github.com/globalsign/mgo/bson"
+	. "gopkg.in/check.v1"
+)
+
+type mixedTagging struct {
+	First  string
+	Second string `bson:"second_field"`
+	Third  string `json:"third_field"`
+	Fourth string `bson:"fourth_field" json:"alternate"`
+}
+
+// TestTaggingFallback checks that tagging fallback can be used/works as expected.
+func (s *S) TestTaggingFallback(c *C) {
+	initial := &mixedTagging{
+		First:  "One",
+		Second: "Two",
+		Third:  "Three",
+		Fourth: "Four",
+	}
+
+	// Take only testing.T, leave only footprints.
+	initialState := bson.JSONTagFallbackState()
+	defer bson.SetJSONTagFallback(initialState)
+
+	// Marshal with the new mode applied.
+	bson.SetJSONTagFallback(true)
+	bsonState, errBSON := bson.Marshal(initial)
+	c.Assert(errBSON, IsNil)
+
+	// Unmarshal into a generic map so that we can pick up the actual field names
+	// selected.
+	target := make(map[string]string)
+	errUnmarshal := bson.Unmarshal(bsonState, target)
+	c.Assert(errUnmarshal, IsNil)
+
+	// No tag, so standard naming
+	_, firstExists := target["first"]
+	c.Assert(firstExists, Equals, true)
+
+	// Just a BSON tag
+	_, secondExists := target["second_field"]
+	c.Assert(secondExists, Equals, true)
+
+	// Just a JSON tag
+	_, thirdExists := target["third_field"]
+	c.Assert(thirdExists, Equals, true)
+
+	// Should marshal 4th as fourth_field (since we have both tags)
+	_, fourthExists := target["fourth_field"]
+	c.Assert(fourthExists, Equals, true)
+}

bson/compatibility.go

@@ -0,0 +1,16 @@
+package bson
+
+// Current state of the JSON tag fallback option. 
+var useJSONTagFallback = false
+
+// SetJSONTagFallback enables or disables the JSON-tag fallback for structure tagging. When this is enabled, structures
+// without BSON tags on a field will fall-back to using the JSON tag (if present).
+func SetJSONTagFallback(state bool) {
+	useJSONTagFallback = state
+}
+
+// JSONTagFallbackState returns the current status of the JSON tag fallback compatability option. See SetJSONTagFallback
+// for more information.
+func JSONTagFallbackState() bool {
+	return useJSONTagFallback
+}