Don't marshal to null for associations after preloading, close #3395

callbacks/preload.go

@@ -110,12 +110,22 @@ func preload(db *gorm.DB, rels []*schema.Relationship, conds []interface{}) {
 	// clean up old values before preloading
 	switch reflectValue.Kind() {
 	case reflect.Struct:
+		switch rel.Type {
+		case schema.HasMany, schema.Many2Many:
+			rel.Field.Set(reflectValue, reflect.MakeSlice(rel.Field.IndirectFieldType, 0, 0).Interface())
+		default:
 			rel.Field.Set(reflectValue, reflect.New(rel.Field.FieldType).Interface())
+		}
 	case reflect.Slice, reflect.Array:
 		for i := 0; i < reflectValue.Len(); i++ {
+			switch rel.Type {
+			case schema.HasMany, schema.Many2Many:
+				rel.Field.Set(reflectValue.Index(i), reflect.MakeSlice(rel.Field.IndirectFieldType, 0, 0).Interface())
+			default:
 				rel.Field.Set(reflectValue.Index(i), reflect.New(rel.Field.FieldType).Interface())
 			}
 		}
+	}
 
 	for i := 0; i < reflectResults.Len(); i++ {
 		elem := reflectResults.Index(i)

tests/preload_test.go

@@ -1,6 +1,8 @@
 package tests_test
 
 import (
+	"encoding/json"
+	"regexp"
 	"sort"
 	"strconv"
 	"testing"
@@ -188,3 +190,25 @@ func TestNestedPreloadWithConds(t *testing.T) {
 		CheckPet(t, *users2[2].Pets[2], *users[2].Pets[2])
 	}
 }
+
+func TestPreloadEmptyData(t *testing.T) {
+	var user = *GetUser("user_without_associations", Config{})
+	DB.Create(&user)
+
+	DB.Preload("Team").Preload("Languages").Preload("Friends").First(&user, "name = ?", user.Name)
+
+	if r, err := json.Marshal(&user); err != nil {
+		t.Errorf("failed to marshal users, got error %v", err)
+	} else if !regexp.MustCompile(`"Team":\[\],"Languages":\[\],"Friends":\[\]`).MatchString(string(r)) {
+		t.Errorf("json marshal is not empty slice, got %v", string(r))
+	}
+
+	var results []User
+	DB.Preload("Team").Preload("Languages").Preload("Friends").Find(&results, "name = ?", user.Name)
+
+	if r, err := json.Marshal(&results); err != nil {
+		t.Errorf("failed to marshal users, got error %v", err)
+	} else if !regexp.MustCompile(`"Team":\[\],"Languages":\[\],"Friends":\[\]`).MatchString(string(r)) {
+		t.Errorf("json marshal is not empty slice, got %v", string(r))
+	}
+}

tests/scan_test.go

@@ -51,11 +51,11 @@ func TestScan(t *testing.T) {
 	DB.Table("users").Select("name, age").Where("id in ?", []uint{user2.ID, user3.ID}).Scan(&results)
 
 	sort.Slice(results, func(i, j int) bool {
-		return strings.Compare(results[i].Name, results[j].Name) < -1
+		return strings.Compare(results[i].Name, results[j].Name) <= -1
 	})
 
 	if len(results) != 2 || results[0].Name != user2.Name || results[1].Name != user3.Name {
-		t.Errorf("Scan into struct map")
+		t.Errorf("Scan into struct map, got %#v", results)
 	}
 }
 
@@ -84,6 +84,10 @@ func TestScanRows(t *testing.T) {
 		results = append(results, result)
 	}
 
+	sort.Slice(results, func(i, j int) bool {
+		return strings.Compare(results[i].Name, results[j].Name) <= -1
+	})
+
 	if !reflect.DeepEqual(results, []Result{{Name: "ScanRowsUser2", Age: 10}, {Name: "ScanRowsUser3", Age: 20}}) {
 		t.Errorf("Should find expected results")
 	}