Fetching Strategy example in repository pattern with pure POCO Entity framework

Question

I'm trying to roll out a strategy pattern with entity framework and the repository pattern using a simple example such as User and Post in which a user has many posts.

From this answer here, I have the following domain:

public interface IUser {
  public Guid UserId { get; set; }
  public string UserName { get; set; }
  public IEnumerable<Post> Posts { get; set; }
}

Add interfaces to support the roles in which you will use the user.

public interface IAddPostsToUser : IUser {
  public void AddPost(Post post);
}

Now my repository looks like this:

public interface IUserRepository {
  User Get<TRole>(Guid userId) where TRole : IUser;
}

Strategy (Where I'm stuck). What do I do with this code? Can I have an example of how to implement this, where do I put this?

public interface IFetchingStrategy<TRole> {
  TRole Fetch(Guid id, IRepository<TRole> role)
}

My basic problem was what was asked in this question. I'd like to be able to get Users without posts and users with posts using the strategy pattern.

Answer 1

If we talk about strategy pattern then IFetchingStrategy must be passed to IUserRepository so I think you should modify Get operation:

public interface IUserRepository 
{   
    User Get<TRole>(Guid userId, IFetchingStrategy<TRole> strategy) where TRole : IUser; 
} 

But I'm not sure how to implement such interfaces with EF.

If we return to your former question, it can also be accomplished this way:

public interface IUserRepository 
{   
    User Get(Guid userId, IEnumerable<Expression<Func<User,object>>> eagerLoading); 
} 

public class UserRepository : IUserRepository
{
    public User Get(Guid userId, IEnumerable<Expression<Func<User,object>>> eagerLoading)
    {
        ObjectQuery<User> query = GetBaseQuery(); // get query somehow, for example from ObjectSet<User>

        if (eagerLoading != null)
        {
            foreach(var expression in eagerLoading)
            {
                // This is not supported out of the box. You need this:
                // http://msmvps.com/blogs/matthieu/archive/2008/06/06/entity-framework-include-with-func-next.aspx
                query = query.Include(expression);
            }
        }

        return query.SingleOrDefault(u => u.Id == userId);
    }
}

You will use the method this way:

User userWithoutPosts = repository.Get(guid, null);
User userWithPosts = repository.Get(guid, new List<Expression<Func<User,object>>>
    {
        u => u.Posts 
    });

But I guess that this implementation works only for first level of navigation properties.

Answer 2

An answer in the following post uses strategy pattern to manipulate a query.

https://codereview.stackexchange.com/questions/3560/is-there-a-better-way-to-do-dynamic-filtering-and-sorting-with-entity-framework