fix(datastore): allow non-empty values in the slice

datastore/datastore.go

@@ -682,7 +682,7 @@ func putMutations(keys []*Key, src interface{}) ([]*pb.Mutation, error) {
 	v := reflect.ValueOf(src)
 	var multiArgType multiArgType
 
-	// If kind is of type slice, return error
+	// If kind is not of type slice, return error
 	if kind := v.Kind(); kind != reflect.Slice {
 		return nil, fmt.Errorf("%w: dst: expected slice got %v", ErrInvalidEntityType, kind.String())
 	}

datastore/save.go

@@ -288,11 +288,17 @@ func saveSliceProperty(props *[]Property, name string, opts saveOpts, v reflect.
 			return err
 		}
 		for _, p := range elemProps {
-			v, ok := values[p.Name]
-			if !ok {
-				return fmt.Errorf("datastore: unexpected property %q in elem %d of slice", p.Name, i)
+			if len(values) != 0 {
+				v, ok := values[p.Name]
+				if !ok {
+					return fmt.Errorf("datastore: unexpected property %q in elem %d of slice", p.Name, i)
+				}
+				values[p.Name] = append(v, p.Value)
+			} else {
+				// This is the first non-empty element
+				values[p.Name] = []interface{}{p.Value}
+				headProps = elemProps
 			}
-			values[p.Name] = append(v, p.Value)
 		}
 	}
 

datastore/util_test.go

@@ -689,6 +689,25 @@ var testCases = []testCase{
 		"",
 		"",
 	},
+	{
+		"omit empty, fields populated, 1st, 2nd and 4th elements in slice are empty",
+		&Omit{
+			A: "a",
+			B: 10,
+			C: true,
+			F: []int{0, 0, 11, 0, 12, 0},
+			S: S{St: "string"},
+		},
+		&PropertyList{
+			Property{Name: "A", Value: "a", NoIndex: false},
+			Property{Name: "Bb", Value: int64(10), NoIndex: false},
+			Property{Name: "C", Value: true, NoIndex: true},
+			Property{Name: "F", Value: []interface{}{int64(11), int64(12)}, NoIndex: false},
+			Property{Name: "St", Value: "string", NoIndex: false},
+		},
+		"",
+		"",
+	},
 	{
 		"omit empty does not propagate",
 		&NoOmits{