EntityFramework Core Lazy Loading returns all related tables - entity-framework

Question

I have 3 tables in my DB:
Countries
States
Cities
I have an API that should return one country with its states.
However, when I attempt this, I end up getting a JSON object with the countries, its stats AND all the cities under each state
My code is something like this (both Eager and Lazy return the same):
//Eager Loading
var countries = await _context.Countries.Include(s=>s.States).ToListAsync(cancellationToken);
//Lazy Loading
var countries = await _context.Countries.ToListAsync(cancellationToken);
How can I only load the country with the states only and leave the cities?

Answer 1

My advice is never return entities. Entities should exist only as long as their DbContext as a representation of the data model. Models used by a view or API serve a different purpose and should be simple, serializable POCOs that EF can populate. This lets them suit solely the data your view/consumer is concerned about. In your case you only care about country and state, not city or other related bits. You may not even need all data about a country or a state. Let EF build a query for just the data needed. This improves performance for your queries, reduces the memory use on server and client, and avoids pitfalls from serialization. (i.e. circular references) Entities should always represent a complete state of an entity. Turning off lazy loading and passing incomplete entity graphs around can easily lead to errors as methods accepting a reference to an entity and faced with a null/empty reference will not know the difference whether that reference was just not loaded, or does not exist.
[Serializable]
public class CountryViewModel
{
public int CountryID { get; set; }
public string CountryName { get; set; }
public IEnumerable<StateViewModel> States { get; set; } = new List<StateViewModel>();
}
[Serializable]
public class StateViewModel
{
public int StateID { get; set; }
public string StateName { get; set; }
}
Then when fetching the countries and states:
var countries = await _context.Countries
.Select(x => new CountryViewModel
{
CountryId = x.CountryId,
CountryName = x.Name,
States = x.States.Select(s => new StateViewModel
{
StateId = s.StateId,
StateName = s.Name
}).ToList()
}).ToListAsync(cancellationToken);
Leveraging Automapper, this can be simplified fairly easily down to:
var countries = await _context.Countries
.ProjectTo<CountryViewModel>()
.ToListAsync(cancellationToken);