使用 Code First 和 EF.Core 引用表以进行有效搜索的正确方法是什么

Question

Fairly new to EF.Core and I'm having some issues as my tables start getting more complex. Here's an example of what I have defined for my classes. Note... there are many more columns and tables than what I have defined below. I've paired them down for brevity.

public class User
{
    public int Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public bool Active { get; set; }
}

Followed by

public class JournalEntry
{
    public int Id { get; set; }
    public int UserId { get; set; }
    public string Details { get; set; }
    public DateTime DateEntered { get; set; }
    public virtual User User { get; set; }
}

I want to be able to issue the following query and INCLUDE the User Table so that I can then populate a ViewModel with columns from the User Table without having to do another lookup and also to sort the data while retrieving it:

public IQueryable<JournalEntry> GetByUser(int userId)
{
    return _DbContext.JournalEntries.Where(j => j.UserId == userId)
                                            .Include(u => u.User)
                                            .OrderBy(u=> u.User.FirstName)
                                                .ThenBy(j => j.DateEntered);
}

My controller would then have something similar to the following:

public IActionResult List(int userId)
{
    var journalEntries = new _dbRepository.GetByUser(userId);
    var myViewModel = new MyViewModel();
    myViewModel.UserName = ($"{journalEntries.User.FirstName} {journalEntries.User.LastName}");
    myViewModel.Entries = journalEntries; 
    etc ....
    return View(myViewModel);
}

I'm loading the user's first and last name in the View Model and whatever other attributes from the various tables that are referenced. The problem that I'm having is that I'm getting errors on the Migration creation " Foreign key constraint may cause cycle or multiple cascade paths. " And of course, if I remove the line reading public virtual User User { get; set; } from the JournalEntry class then the problem goes away (as one would expect).

I believe that the way I'm doing the models is incorrect. What would be the recommended way that I should code these models? I've heard of "lazy loading". Is that what I should be moving towards?

Thanks a bunch.

--- Val

Answer 1

Your query returns an IQueryable<JournalEntry> not a JournalEntry . Change the code to get the user details from the first object:

var myViewModel.UserName = ($"{journalEntries.First().User.FirstName} {journalEntries.First().User.LastName}");

In the line above I'm calling First() on your journal entries collection and that would have a User . Then I can access FirstName and LastName .

Also, don't bother with LazyLoading since you are learning. It could cause select n+1 issues if used incorrectly