feat: improvements in entity domain logic

domain/usecase/loan/mock/loan.go

@@ -5,10 +5,9 @@
 package mock
 
 import (
-	reflect "reflect"
-
 	entity "github.com/eminetto/clean-architecture-go-v2/domain/entity"
 	gomock "github.com/golang/mock/gomock"
+	reflect "reflect"
 )
 
 // MockUseCase is a mock of UseCase interface

domain/usecase/loan/service.go

@@ -35,10 +35,6 @@ func (s *Service) Borrow(u *entity.User, b *entity.Book) error {
 		return domain.ErrNotEnoughBooks
 	}
 
-	_, err = u.GetBook(b.ID)
-	if err == nil {
-		return domain.ErrBookAlreadyBorrowed
-	}
 	err = u.AddBook(b.ID)
 	if err != nil {
 		return err