一对一使用ICollection和Entity Framework，以及如何提高性能

Question

I recently started to work with EF and MVC in .Net Web Applications, and I've encountered an issue, and wanted to see what your thoughts were and whether you would be able to point me in the right direction.

So, I've got two classes: DataGroup and DataElements. A DataElement can be in one DataGroup or none. I've used fluent API in the DataContext to achieve this:

    modelBuilder.Entity<DataGroup>()
            .HasMany(dg => dg.DataElements)
            .WithOptional() 
            .HasForeignKey(de => de.DataGroup_Id);

In my DataGroup class I've got the following:

    public virtual ICollection<DataElement> DataElements { get; set; }

    [NotMapped]
    public ICollection<DataElement> RecentDataElements //returns the last 15 data elements
    {
        get
        {
            return DataElements.OrderByDescending(de => de.GeneratedDateTime).Take(15).Reverse().ToList();
        }
    }

    [NotMapped]
    public DataElement LatestDataElement //returns the latest data element (if any)
    {
        return DataElements.OrderByDescending(de => de.GeneratedDateTime).FirstOrDefault();
    }

So the issue I have with the code above is that when I call LatestDataElement or RecentDataElements, I end up waiting quite a bit.

I believe this is because the DataElements property in the DataGroup class is an ICollection and RecentDataElements and LatestDataElement end up causing EF to load all the DataElements into memory before sorting and returning a sub-set (there could be thousands of DataElements in a DataGroup).

Is there a way to make this more efficient?

I've toyed with the idea of querying the datacontext directly rather than using the DataElements property, but I wanted to see if there were any other options I should consider. I've been told by colleagues that it would be bad practice to put the datacontext in a model (whether they're right or wrong is a different issue).

Thank you for your help and advice. It's much appreciated :)

Answer 1

In short: No .

As the name of the NotMapped attribute already suggests, this is a property linq-to-entities has no knowledge of whatsoever and it's impossible to populate it by filtering on the database. What I would do in your place is remove the Notmapped properties from your model all together and create a viewmodel:

public class DataGroupViewModel
{
    public DataGroup DataGroup {get; set;}

    public ICollection<DataGroup> RecentDataElements {get; set;}
}

And use projection in your query to populate this view:

var result = ctx.DataGroups.Where(...).Select(d => new DataGroupViewModel
{
    DataGroup = d;
    RecentDataElements = d.DataElements.OrderByDescending(de => de.GeneratedDateTime)
       .FirstOrDefault();
}

This of course forces you to create another model but it's the cleanest and fastest way to do it. Also your colleagues are right, it is a bad way to do db calls in your model, a model is a container for data, nothing more than that, it shouldn't have logic inside it to query the database.

To make it a bit cleaner you can write some extension methods you can reuse to get the models:

Func<IQueryable<DataGroup>,IEnumerable<DataGroupViewModel>> GetDataGroupDTO = 
   d => d.Select(dt => new DataGroupViewModel 
   {
      DataGroup = dt;
      RecentDataElements = dt.DataElements.OrderByDescending(de => de.GeneratedDateTime)
       .FirstOrDefault();
   }

Then you can write your query more clean:

IEnumerable<DataGroupViewModel> result = ctx.DataGroups.Where(...).GetDataGroupDTO();