feat: Enhance user update handling and introduce NullString type for optional fields

backend/internal/models/user.go

@@ -35,12 +35,12 @@ const (
 // User represents a user in the system
 type User struct {
 	EntityBase
-	Email      string      `gorm:"column:email;unique;not null"`
-	Salt       string      `gorm:"column:salt;not null;type:varchar(64)"`  // Base64-encoded Salt
-	Hash       string      `gorm:"column:hash;not null;type:varchar(128)"` // Base64-encoded Hash
-	Role       string      `gorm:"column:role;not null;default:'user'"`
-	CompanyID  ULIDWrapper `gorm:"column:company_id;type:bytea;not null;index"`
-	HourlyRate float64     `gorm:"column:hourly_rate;not null;default:0"`
+	Email      string       `gorm:"column:email;unique;not null"`
+	Salt       string       `gorm:"column:salt;not null;type:varchar(64)"`  // Base64-encoded Salt
+	Hash       string       `gorm:"column:hash;not null;type:varchar(128)"` // Base64-encoded Hash
+	Role       string       `gorm:"column:role;not null;default:'user'"`
+	CompanyID  *ULIDWrapper `gorm:"column:company_id;type:bytea;index"`
+	HourlyRate float64      `gorm:"column:hourly_rate;not null;default:0"`
 
 	// Relationship for Eager Loading
 	Company *Company `gorm:"foreignKey:CompanyID"`
@@ -335,10 +335,22 @@ func GetAllUsers(ctx context.Context) ([]User, error) {
 	return users, nil
 }
 
+// getCompanyCondition builds the company condition for queries
+func getCompanyCondition(companyID *ULIDWrapper) func(db *gorm.DB) *gorm.DB {
+	return func(db *gorm.DB) *gorm.DB {
+		if companyID == nil {
+			return db.Where("company_id IS NULL")
+		}
+		return db.Where("company_id = ?", *companyID)
+	}
+}
+
 // GetUsersByCompanyID returns all users of a company
 func GetUsersByCompanyID(ctx context.Context, companyID ULIDWrapper) ([]User, error) {
 	var users []User
-	result := GetEngine(ctx).Where("company_id = ?", companyID).Find(&users)
+	// Apply the dynamic company condition
+	condition := getCompanyCondition(&companyID)
+	result := GetEngine(ctx).Scopes(condition).Find(&users)
 	if result.Error != nil {
 		return nil, result.Error
 	}
@@ -386,7 +398,7 @@ func CreateUser(ctx context.Context, create UserCreate) (*User, error) {
 			Salt:       pwData.Salt,
 			Hash:       pwData.Hash,
 			Role:       create.Role,
-			CompanyID:  create.CompanyID,
+			CompanyID:  &create.CompanyID,
 			HourlyRate: create.HourlyRate,
 		}
 
@@ -435,7 +447,7 @@ func UpdateUser(ctx context.Context, update UserUpdate) (*User, error) {
 		}
 
 		// If CompanyID is updated, check if it exists
-		if update.CompanyID != nil && update.CompanyID.Compare(user.CompanyID) != 0 {
+		if update.CompanyID != nil && (user.CompanyID == nil || update.CompanyID.Compare(*user.CompanyID) != 0) {
 			var companyCount int64
 			if err := tx.Model(&Company{}).Where("id = ?", *update.CompanyID).Count(&companyCount).Error; err != nil {
 				return fmt.Errorf("error checking company: %w", err)