服务应具有多少个GET方法？

Question

What are the best practices for writing a service layer methods that retrieves data from repository?

Let's say we have two models: Team and User (user is part of a team):

public class User {
    public int Id { get; set; }
    public string Name { get; set; }
    public int TeamId { get; set; }
    public virtual Team Team { get; set; }
    public bool Active { get; set; }
}

public class Team {
    public int Id { get; set; }
    public string Name { get; set; }
    public bool Active { get; set; }
}

If I want to write service to retrieve user data from repository by various conditions, do I have to write multiple methods to get a user eg getAll , getAllByName , getAllActiveByName , getAllActiveByTeamId , getAllActiveByNameAndTeamId , etc?

public IEnumerable<User> GetAll()
{
    return _repository.GetAll();
}

public IEnumerable<User> GetAllActiveByName(string name)
{
    return _repository.GetBy(u => u.Name == name && u.Active);
}

public IEnumerable<User> GetAllActiveByNameAndTeamId(string name, int teamId)
{
    return _repository.GetBy(u => u.Name == name && u.Active && u.TeamId == teamId);
}

These are just simple examples but in real life we can end up having tens of service methods for different scenarios, when models are more complex.

Or maybe it is better to have one GetBy method that will return users based on the provided filter? I use Generic Repository Pattern and I could use GetBy method when implementing GetBy service method:

public IEnumerable<User> GetBy(Expression<Func<User, object>>filter )
{
    return _usersRepository.GetBy(filter);
}

Having this I would not have to write tens of "duplicated" methods for all the scenarios. Then it would be controller responsibility to set the filter:

public ViewResult Index(int teamId = 0){
    //[...]
    var users = _usersService.GetBy(u => u.IsActive && u.teamId == teamId);
    //[...]
}

Any thoughts on that?

Answer 1

I'm of the opinion that you should have as many query methods as you have scenarios.

In that way you can optimize individual queries by for example using a precalculated view. Some of your queries might use eager loading, other might use lazy loading...

Also, if you always return IQueryable how are you going to test it? Your service will have only one method GetAll and is so anemic that you can just get rid of it and use repository directly in the controller.

Another argument against GetAll is that any one can execute any query in the UI!

Consider reading about CQRS.

Answer 2

Or maybe it is better to have one getAll method that will return only active users and then use lambda expression in the controller?

No. This kind of query will be good only for static data that too in-memory. Say you have some application level data and it is not going to change for certain time, then instead of querying it everytime, you getall for first time and then put in local server cache. Then use it for next simultaneous requests. But this approach is not going to for heavily changing dynamic data. Also performance of this query depends on number of records it is returning, so sometimes it might give very bad performance.

do I have to write multiple methods to get a user eg getAll, getAllByName, getAllActiveByName, getAllActiveByTeamId, getAllActiveByNameAndTeamId, etc?

Thats better. This approach will give you load on demand freedom, that means load necessary data when it is required, instead of getting all data and discarding it.