Question

Just need to clarify this one, If I have the below interface

public interface IRepository<T>
{
    T Add(T entity);
}

when implementing it, does checking for duplication if entity is already existing before persist it is still a job of the Repository, or it should handle some where else?

Answer 1

Yes – I recommend doing these checks in the repository.

Long answer: The term “repository” is a bit vague, but it is used more and more as the name of the persistence abstraction layer. The name is nice, but does not say too much: If you take Asp.Net MVC as an example, the sample apps, like Neirds dinner and alike, or codeplex projects encapsulate data access by the repository class. If such layer is implemented with a relational database for instancce, the primary keys of the tables will not allow duplicate entries, which means that in this case the repository implementation will throw an exception if 2 entries with the same key are inserted. So in other words, a RDBMS-implementation of a repository will quite always due this check, you wont be able to avoid it. So to make the behavior of repostories out there in the world most similar and to avoid surprises, lets all of them do this check.

It is a second question whether you should maintain in the business logic already that your Add() method is not alled with an entry that already exists. Sometimes it makes good sense to resolve this only at a single point, the database for instance, due to concurrency issues or savings of roundtrips. On the other hand it is for instance nice to tell the user as soon as possible that a username is already taken. So this depends.

have a nice day