Entity Framework Core 5.0 - Many to many select query - entity-framework-core

Question

I am trying to get a single User, with a list of Items, mapped with a many-to-many entity UserItems. However, I am unable to retrieve the mapped Items due to to an error that I'm unable to solve (error at bottom of question). Here is my code:
public class User
{
public int Id { get; set; }
public ICollection<UserItem> UserItems { get; set; }
}
public class Item
{
public int Id { get; set; }
public ICollection<UserItem> UserItems { get; set; }
}
public class UserItem
{
public int Id { get; set; }
public int UserId { get; set; }
public User User { get; set; }
public int ItemId { get; set; }
public Item Item { get; set; }
public int Quantity { get; set; }
}
The UserItem class configuration has the following relationships defined:
builder.HasOne(x => x.User)
.WithMany(x => x.UserItems)
.HasForeignKey(x => x.UserId)
.OnDelete(DeleteBehavior.ClientCascade);
builder.HasOne(x => x.Item)
.WithMany(x => x.UserItems)
.HasForeignKey(x => x.ItemId)
.OnDelete(DeleteBehavior.ClientCascade);
I have the following generic repo with this method:
public class GenericRepository<T> : where T : class
{
private readonly DbContext _context;
public GenericRepository(DbContext context) => _context = context;
public T Get(Expression<Func<T, bool>> where, params Expression<Func<T, object>>[] navigationProperties)
{
IQueryable<T> query = _context.Set<T>();
query = navigationProperties.Aggregate(query, (current, property) => current.Include(property));
var entity = query.FirstOrDefault(where);
return entity;
}
}
However, when I try to run the code, I get an error on the Select(x => x.Item):
var user = repo.Get(x => x.Id == 1, x => x.UserItems.Select(y => y.Item));
Error:
System.InvalidOperationException: 'The expression 'x.UserItems.AsQueryable().Select(y => y.Item)' is invalid inside an 'Include' operation, since it does not represent a property access: 't => t.MyProperty'. To target navigations declared on derived types, use casting ('t => ((Derived)t).MyProperty') or the 'as' operator ('t => (t as Derived).MyProperty'). Collection navigation access can be filtered by composing Where, OrderBy(Descending), ThenBy(Descending), Skip or Take operations. For more information on including related data, see http://go.microsoft.com/fwlink/?LinkID=746393.'
What am I doing wrong, this seems to work for my other projects?

Answer 1

This error Occurs because you are not passing in a navigation property (x.UserItems would be a navigation property) but rather something you want to do with the navigation property. UserItems.Select(y => y.Item) is not a property of x because Select() is a function and therefore it cannot be included.
What you are trying to do (I assume it is including UserItems and also the corresponding Items) is not going to work with your current implementation of the repository. To include navigation properties of navigation properties .ThenInclude() must be used instead of .Include() which works only for navigation properties directly defined on the Entity the DbSet is created for.
But apart from your question I would suggest not to use such an generic implementation of Repository. The main benefit from using reposiories is to separarte code related to loading and storing of entities from the rest of your code. In your case if the consumer of repository knows that navigation properties must be included and that he has to provide them - then what is the point of having a repository at all? Then the consumer again cares about database specific code which makes having a repository unneccessary. I would recommend just making a conrete "UserRepository" which can only be used to retrieve users and explicitly includes the needed properties.