fix(scan): update Scan function to reset structs to zero values for each scan (#7061)

Co-authored-by: waleed.masoom <waleed.masoom@wheniwork.com>

scan.go

@@ -331,6 +331,9 @@ func Scan(rows Rows, db *DB, mode ScanMode) {
 			}
 		case reflect.Struct, reflect.Ptr:
 			if initialized || rows.Next() {
+				if mode == ScanInitialized && reflectValue.Kind() == reflect.Struct {
+					db.Statement.ReflectValue.Set(reflect.Zero(reflectValue.Type()))
+				}
 				db.scanIntoStruct(rows, reflectValue, values, fields, joinFields)
 			}
 		default:

tests/scan_test.go

@@ -5,6 +5,7 @@ import (
 	"sort"
 	"strings"
 	"testing"
+	"time"
 
 	"gorm.io/gorm"
 	. "gorm.io/gorm/utils/tests"
@@ -126,7 +127,7 @@ func TestScanRows(t *testing.T) {
 
 	rows, err := DB.Table("users").Where("name = ? or name = ?", user2.Name, user3.Name).Select("name, age").Rows()
 	if err != nil {
-		t.Errorf("Not error should happen, got %v", err)
+		t.Errorf("No error should happen, got %v", err)
 	}
 
 	type Result struct {
@@ -148,7 +149,7 @@ func TestScanRows(t *testing.T) {
 	})
 
 	if !reflect.DeepEqual(results, []Result{{Name: "ScanRowsUser2", Age: 10}, {Name: "ScanRowsUser3", Age: 20}}) {
-		t.Errorf("Should find expected results")
+		t.Errorf("Should find expected results, got %+v", results)
 	}
 
 	var ages int
@@ -158,7 +159,105 @@ func TestScanRows(t *testing.T) {
 
 	var name string
 	if err := DB.Table("users").Where("name = ?", user2.Name).Select("name").Scan(&name).Error; err != nil || name != user2.Name {
-		t.Fatalf("failed to scan ages, got error %v, ages: %v", err, name)
+		t.Fatalf("failed to scan name, got error %v, name: %v", err, name)
+	}
+}
+
+func TestScanRowsNullValuesScanToFieldDefault(t *testing.T) {
+	DB.Save(&User{})
+
+	rows, err := DB.Table("users").
+		Select(`
+			NULL AS bool_field,
+			NULL AS int_field,
+			NULL AS int8_field,
+			NULL AS int16_field,
+			NULL AS int32_field,
+			NULL AS int64_field,
+			NULL AS uint_field,
+			NULL AS uint8_field,
+			NULL AS uint16_field,
+			NULL AS uint32_field,
+			NULL AS uint64_field,
+			NULL AS float32_field,
+			NULL AS float64_field,
+			NULL AS string_field,
+			NULL AS time_field,
+			NULL AS time_ptr_field,
+			NULL AS embedded_int_field,
+			NULL AS nested_embedded_int_field,
+			NULL AS embedded_ptr_int_field
+        `).Rows()
+	if err != nil {
+		t.Errorf("No error should happen, got %v", err)
+	}
+
+	type NestedEmbeddedStruct struct {
+		NestedEmbeddedIntField            int
+		NestedEmbeddedIntFieldWithDefault int `gorm:"default:2"`
+	}
+
+	type EmbeddedStruct struct {
+		EmbeddedIntField     int
+		NestedEmbeddedStruct `gorm:"embedded"`
+	}
+
+	type EmbeddedPtrStruct struct {
+		EmbeddedPtrIntField   int
+		*NestedEmbeddedStruct `gorm:"embedded"`
+	}
+
+	type Result struct {
+		BoolField          bool
+		IntField           int
+		Int8Field          int8
+		Int16Field         int16
+		Int32Field         int32
+		Int64Field         int64
+		UIntField          uint
+		UInt8Field         uint8
+		UInt16Field        uint16
+		UInt32Field        uint32
+		UInt64Field        uint64
+		Float32Field       float32
+		Float64Field       float64
+		StringField        string
+		TimeField          time.Time
+		TimePtrField       *time.Time
+		EmbeddedStruct     `gorm:"embedded"`
+		*EmbeddedPtrStruct `gorm:"embedded"`
+	}
+
+	currTime := time.Now()
+	reusedVar := Result{
+		BoolField:         true,
+		IntField:          1,
+		Int8Field:         1,
+		Int16Field:        1,
+		Int32Field:        1,
+		Int64Field:        1,
+		UIntField:         1,
+		UInt8Field:        1,
+		UInt16Field:       1,
+		UInt32Field:       1,
+		UInt64Field:       1,
+		Float32Field:      1.1,
+		Float64Field:      1.1,
+		StringField:       "hello",
+		TimeField:         currTime,
+		TimePtrField:      &currTime,
+		EmbeddedStruct:    EmbeddedStruct{EmbeddedIntField: 1, NestedEmbeddedStruct: NestedEmbeddedStruct{NestedEmbeddedIntField: 1, NestedEmbeddedIntFieldWithDefault: 2}},
+		EmbeddedPtrStruct: &EmbeddedPtrStruct{EmbeddedPtrIntField: 1, NestedEmbeddedStruct: &NestedEmbeddedStruct{NestedEmbeddedIntField: 1, NestedEmbeddedIntFieldWithDefault: 2}},
+	}
+
+	for rows.Next() {
+		if err := DB.ScanRows(rows, &reusedVar); err != nil {
+			t.Errorf("should get no error, but got %v", err)
+		}
+	}
+
+	if !reflect.DeepEqual(reusedVar, Result{}) {
+		t.Errorf("Should find zero values in struct fields, got %+v\n", reusedVar)
 	}
 }