fix: skip append relation field to default db value (#5885)

* fix: relation field returning

* chore: gofumpt style

schema/schema.go

@@ -230,7 +230,7 @@ func ParseWithSpecialTableName(dest interface{}, cacheStore *sync.Map, namer Nam
 	}
 
 	for _, field := range schema.Fields {
-		if field.HasDefaultValue && field.DefaultValueInterface == nil {
+		if field.DataType != "" && field.HasDefaultValue && field.DefaultValueInterface == nil {
 			schema.FieldsWithDefaultDBValue = append(schema.FieldsWithDefaultDBValue, field)
 		}
 	}

tests/associations_belongs_to_test.go

@@ -3,6 +3,7 @@ package tests_test
 import (
 	"testing"
 
+	"gorm.io/gorm"
 	. "gorm.io/gorm/utils/tests"
 )
 
@@ -224,3 +225,28 @@ func TestBelongsToAssociationForSlice(t *testing.T) {
 	AssertAssociationCount(t, users[0], "Company", 0, "After Delete")
 	AssertAssociationCount(t, users[1], "Company", 1, "After other user Delete")
 }
+
+func TestBelongsToDefaultValue(t *testing.T) {
+	type Org struct {
+		ID string
+	}
+	type BelongsToUser struct {
+		OrgID string
+		Org   Org `gorm:"default:NULL"`
+	}
+
+	tx := DB.Session(&gorm.Session{})
+	tx.Config.DisableForeignKeyConstraintWhenMigrating = true
+	AssertEqual(t, DB.Config.DisableForeignKeyConstraintWhenMigrating, false)
+
+	tx.Migrator().DropTable(&BelongsToUser{}, &Org{})
+	tx.AutoMigrate(&BelongsToUser{}, &Org{})
+
+	user := &BelongsToUser{
+		Org: Org{
+			ID: "BelongsToUser_Org_1",
+		},
+	}
+	err := DB.Create(&user).Error
+	AssertEqual(t, err, nil)
+}