GODRIVER-1542 Add DateTime.UnmarshalJSON (#355)

Author: Divjot Arora

bson/primitive/primitive.go

@@ -40,11 +40,32 @@ type Undefined struct{}
 // DateTime represents the BSON datetime value.
 type DateTime int64
 
+var _ json.Marshaler = DateTime(0)
+var _ json.Unmarshaler = (*DateTime)(nil)
+
 // MarshalJSON marshal to time type
 func (d DateTime) MarshalJSON() ([]byte, error) {
 	return json.Marshal(d.Time())
 }
 
+// UnmarshalJSON creates a primitive.DateTime from a JSON string.
+func (d *DateTime) UnmarshalJSON(data []byte) error {
+	// Ignore "null" to keep parity with the time.Time type and the standard library. Decoding "null" into a non-pointer
+	// DateTime field will leave the field unchanged. For pointer values, the encoding/json will set the pointer to nil
+	// and will not defer to the UnmarshalJSON hook.
+	if string(data) == "null" {
+		return nil
+	}
+
+	var tempTime time.Time
+	if err := json.Unmarshal(data, &tempTime); err != nil {
+		return err
+	}
+
+	*d = NewDateTimeFromTime(tempTime)
+	return nil
+}
+
 // Time returns the date as a time type.
 func (d DateTime) Time() time.Time {
 	return time.Unix(int64(d)/1000, int64(d)%1000*1000000)

bson/primitive/primitive_test.go

@@ -7,9 +7,11 @@
 package primitive
 
 import (
+	"encoding/json"
 	"testing"
 
 	"github.com/stretchr/testify/require"
+	"go.mongodb.org/mongo-driver/internal/testutil/assert"
 )
 
 // The same interface as bsoncodec.Zeroer implemented for tests.
@@ -78,3 +80,26 @@ func TestRegexCompare(t *testing.T) {
 		})
 	}
 }
+
+func TestDateTime(t *testing.T) {
+	t.Run("json", func(t *testing.T) {
+		t.Run("round trip", func(t *testing.T) {
+			original := DateTime(1000)
+			jsonBytes, err := json.Marshal(original)
+			assert.Nil(t, err, "Marshal error: %v", err)
+
+			var unmarshalled DateTime
+			err = json.Unmarshal(jsonBytes, &unmarshalled)
+			assert.Nil(t, err, "Unmarshal error: %v", err)
+
+			assert.Equal(t, original, unmarshalled, "expected DateTime %v, got %v", original, unmarshalled)
+		})
+		t.Run("decode null", func(t *testing.T) {
+			jsonBytes := []byte("null")
+			var dt DateTime
+			err := json.Unmarshal(jsonBytes, &dt)
+			assert.Nil(t, err, "Unmarshal error: %v", err)
+			assert.Equal(t, DateTime(0), dt, "expected DateTime value to be 0, got %v", dt)
+		})
+	})
+}