MVC，实体框架-是否有更好的方法来过滤项目？

Question

I'm using a separate class to keep the filter options:

    public class FilterViewModel
    {
        public string UserName { get; set; }

        public int? TownId { get; set; }
        ...
     }

In the action I use a predicate which takes the filter as a parameter.That way the "Where" method returns IEnumerable instaed of IQuerable:

    public ActionResult FilterProfiles(FilterViewModel filter)
    {
       var profiles = this.Data.Profiles.All()
            .Where(Predicate(filter)) 
            .OrderBy(p => p.ProfileUser.UserName).AsQueryable()
            .Project()
            .To<ProfileViewModel>()
            .ToList();
      }

    private static Func<UserProfile, bool> Predicate(FilterViewModel f)
    {
       return p => (CompareFilter(p, f));
    }

    private static bool CompareFilter(UserProfile profile, FilterViewModel filter)
    {
        if (filter.FirstName != null)
        {
            if (profile.FirstName != null)
            {
                if (profile.FirstName.CompareTo(filter.FirstName) != 0)
                {
                    return false;
                }
            }
       ...
      }

This worked until in ProfileViewModel I implemented database DateTime? operation in the mapping:

    public class ProfileViewModel : IHaveCustomMappings
    {
        ...

        public bool IsUserOnline { get; set; }

        ...

        public void CreateMappings(IConfiguration configuration)
        {
            configuration.CreateMap<UserProfile, ProfileViewModel>()
                .ForMember(m => m.IsUserOnline, opt => opt.MapFrom(p =>
                   DbFunctions.DiffMinutes(p.ProfileUser.LastActionTime, DateTime.Now) < 5 ? true : false))
        }
    }

Then on the "Where" method in the action an error appears:

[NotSupportedException: This function can only be invoked from LINQ to Entities.] System.Data.Entity.DbFunctions.DiffMinutes(Nullable 1 timeValue1, Nullable 1 timeValue2) +56

I'm also wondering does in this case, IEnumerable "Where" copys all the database items in the memory and then filters them?

Thanks in advance!

Answer 1

EF expects Expression<Func<T,bool>> , but you are returning Func<T,bool>

You have to change it to following

private static Expression<Func<UserProfile, bool>> 
    Predicate(FilterViewModel f)
{
   return CompareFilter(f));
}

private static Expression<Func<UserProfile, bool>>
    CompareFilter(FilterViewModel filter)
{
    if (filter.FirstName != null)
    {
       return p => p.FirstName == filter.FirstName;
    }
   ...

   // this means nothing to compare, 
   // return all records...
   return p => true;
}

In case if you want to apply multiple filters, then you will have to filter IQueryable itself.

private static IQueryable<UserProfile> 
    Predicate(IQueryable<UserProfile> q, FilterViewModel f)
{

    if (filter.FirstName != null)
    {

        q = q.Where( p => p.FirstName == filter.FirstName );
    }

    if (filter.LastName != null)
    {

        q = q.Where( p => p.LastName == filter.LastName );
    }
   ...

   // return all records...
   return q;
}

In order to run SQL and related operations on server, you have to apply filter on IQueryable which will be executed on server, instead of loading all them locally and then try to filter it.