Entity Framework: Fetch multiple collections with filters applied

问题

Giving the next model:

public class User{
    public int IdUser { get; set;}
    public virtual ICollection<Project> Projects { get; set;}
    public virtual ICollection<Exam> Exams { get; set;}
}

public class Project{
    public int IdProject { get; set;}
    public bool Current { get; set;}
    public virtual User { get; set;}
}

public class Exam{
    public int IdExam { get; set;}
    public int score { get; set;}
    public virtual User { get; set;}
}

I need to get the projects with current=true and exams with score greater than 4 from a given user.

When i need to filter a navigation property, for avoiding bringing all the records and apply the filter in memory, i do the next:

IQueryable<Project> = context.Entry(user).Collection(x => x.Projects).Query().Where(n => n.Current).ToList();

On that way, i bring from the database only the current projects. Avoiding the other way of retrieving all the projects to memory and then applying the filter on memory.

So now, i want to do the same (bring only the records that matters), but i don't know how can i do that when i have more than one collection.

Can you help me? Thanks!

回答1:

Your repository is going to fill up very rapidly with methods that return entities with different types of filters on the navigation properties. Perhaps you should have a method that looks like this:

   public GetUser(int userid, 
                  Expression<System.Func<Project, System.Boolean>> projectFilter,
                  Expression<System.Func<Exam, System.Boolean>> examFilter)
    {
       var user = context.Users.Find(userid); 

       context.Entry(user)
              .Collection(c => c.Projects)
              .Query()
              .Where(projectFilter)
              .Load(); 

       context.Entry(user)
              .Collection(c => c.Exams)
              .Query()
              .Where(examFilter)
              .Load();

       return user
    }

And you call it like this:

var userincludingCurrentProjectsAndExamsWithScoresLessThan4 = 
                         userRepo.GetUser(id, p => p.Current, e => e.Score > 4)

And don't forget to remove the virtual keywords from the collections or lazy loading will pull the whole collection out of the database before the filter is applied:

public class User
{
    public int IdUser { get; set;}
    public ICollection<Project> Projects { get; set;}
    public ICollection<Exam> Exams { get; set;}
}

EDIT I have to say that this technique of partially populating navigation properties smells a bit funny to me. I think what most people do is project the data into DTOs or ViewModels to achieve what you are trying to do. Then the DTO's can have property names that make more sense. Something like this:

var query = from f in context.Foos.Include(x => x.Bars).Include(y => y.Bazs)
            select new FooDTO
            {
                ID = f.ID,
                Toms = f.Bars.Where(b => b.Name == "Tom").ToList(),
                Dicks = f.Bazs.Where(b => b.Name == "Dick").ToList()
            };

string sql = query.ToString();//useful in debugger. Remove once satisfied.

But if you really want to project into an entity rather than a DTO you could do this:

var usersWithTomsAndDicksOohErr = 
(from f in context.Foos.Include(x => x.Bars).Include(y => y.Bazs)
    select new //anonymous. You can't construct entities using LINQ to Entities
    {
        ID = f.ID,
        Toms = f.Bars.Where(b => b.Name == "Tom").ToList(),
        Dicks = f.Bazs.Where(b => b.Name == "Dick").ToList()
    })
    .AsEnumerable()//moves only the data you want into memory
    .Select(x => new Foo//now you can construct the entity using Linq to Objects
    {
       ID = x.ID,
       Bars = x.Toms,
       Bazs = x.Dicks
    });

回答2:

what about:

var q = context.Set<Project>().Where(x => x.Current && x.User.Exams.Where(y => y.score > 4).Count() > 0)

or may be

var q = context.Set<Project>().Where(x => x.Current && x.User.Exams.Any(y => y.score > 4))

来源：https://stackoverflow.com/questions/20524311/entity-framework-fetch-multiple-collections-with-filters-applied