Question

What should be the right practice to handle duplicate linq queries to Database?
By duplicate queries, I mean such queries that recurs in my code.

Example:

I notice that in my code recurs following query:

dataContext.Users.Where(u =>u.IsActive && u.IsAdmin).OrderByDescent(u => u.Name)

Where should I place the method that contains above query. Should it be UserRepository(in this manner, I will probably end with god class which is bad practice)? or should I create class architecture to handle this.

I wonder what are your thoughts about this. And what class structure in your opinion will be most generic and vulnerable to reuse.

I have in mind that, I can use a store procedure as a place for recurring queries, but I don’t want to go that way.

Answer 1

Yes, it can be in the UserRepository, but you can also build up an Godclass for standardized queries (Create, Delete, and so on) and take use of an Service Layer or Extensions to Filter the results from GetAll() Method in the UserRepository.

Update
I’m trying to show it a little bit more detailed.

1. Build an generic repository that holds all your statements to retrieve/store your data.
2. Create an service layer that consume from/send to the repository.
3. Add extension methods to filter for specific results. (featured with c# 3.0)

Some code:

// Repository (non-generic)
private IQueryable<User> GetUser()
{
  var results = from u in _db.Users
                select new User
                {
                  ID = u.ID,
                  Username = u.Username,
                  // and so on
                };

  return results;
}


//Service Layer  
private User GetUser(string Username)
{
   User u = _repository.GetUser().WithUsername(username)
                                 .SingleOrDefault();
} 

// Extension Method
public static IQueryable<User> WithUsername(this IQueryable<User> qry, string username)
{
  return from u in qry
         where u.Username == username
         select u;
}

This approach is related to Rob Conerys MVC StoreFront Preview. The implementation maybe vary in other projects, but this is one way to do it.