fix(dgraph): Fix empty string response for zero datetime

query/outputnode.go

@@ -331,9 +331,8 @@ func (enc *encoder) IsEmpty(fj fastJsonNode) bool {
 }
 
 var (
-	boolTrue    = []byte("true")
-	boolFalse   = []byte("false")
-	emptyString = []byte(`""`)
+	boolTrue  = []byte("true")
+	boolFalse = []byte("false")
 
 	// Below variables are used in stringJsonMarshal function.
 	bufferPool = sync.Pool{
@@ -465,11 +464,7 @@ func valToBytes(v types.Val) ([]byte, error) {
 		}
 		return boolFalse, nil
 	case types.DateTimeID:
-		// Return empty string instead of zero-time value string - issue#3166
 		t := v.Value.(time.Time)
-		if t.IsZero() {
-			return emptyString, nil
-		}
 		return t.MarshalJSON()
 	case types.GeoID:
 		return geojson.Marshal(v.Value.(geom.T))

systest/mutations_test.go

@@ -102,6 +102,7 @@ func TestSystem(t *testing.T) {
 	t.Run("overwrite uid predicates", wrap(OverwriteUidPredicates))
 	t.Run("overwrite uid predicates reverse index", wrap(OverwriteUidPredicatesReverse))
 	t.Run("delete and query same txn", wrap(DeleteAndQuerySameTxn))
+	t.Run("add and query zero datetime value", wrap(AddAndQueryZeroTimeValue))
 }
 
 func FacetJsonInputSupportsAnyOfTerms(t *testing.T, c *dgo.Dgraph) {
@@ -2395,3 +2396,37 @@ func DeleteAndQuerySameTxn(t *testing.T, c *dgo.Dgraph) {
 	testutil.CompareJSON(t, `{"me":[{"name":"Alice"}]}`,
 		string(resp.GetJson()))
 }
+
+func AddAndQueryZeroTimeValue(t *testing.T, c *dgo.Dgraph) {
+	ctx := context.Background()
+
+	op := &api.Operation{Schema: `val: datetime .`}
+	require.NoError(t, c.Alter(ctx, op))
+
+	txn := c.NewTxn()
+	_, err := txn.Mutate(ctx, &api.Mutation{
+		SetNquads: []byte(`
+			_:value <val> "0000-01-01T00:00:00Z" .
+		`),
+	})
+	require.NoError(t, err)
+	require.NoError(t, txn.Commit(ctx))
+
+	const datetimeQuery = `
+	{
+		q(func: has(val)) {
+			val
+		}
+	}`
+
+	txn = c.NewTxn()
+	resp, err := txn.Query(ctx, datetimeQuery)
+	require.NoError(t, err)
+	testutil.CompareJSON(t, `{
+		"q": [
+		  {
+			"val": "0000-01-01T00:00:00Z"
+		  }
+		]
+	  }`, string(resp.Json))
+}