Question

I have a problem in Entity-Framework, using Code-First, that I couldn’t solve.

Having entities of the type

public class Product {
  public int ID {get; set; }
  public virtual ICollection<Category> Categories { get; set; }
}

public class Category {
  public int ID {get; set;}
  public virtual ICollection<Product> Products { get; set; }
  // rest omitted
}

in my database, i try to get all Products that have at least one Category from a list of given Categories. I need an Expression as this expression is combined with other expressions later.

Ie. i tried:

var searchFor = new List<Category>{...};
var expression = product => product.Categories.Any(cat => searchFor.Contains(cat))

Executing this later against a DbContext

context.Products.Where(expression).ToList();

creates an exception stating mainly that This context supports primitive types only.

Changing it to

var expression = product => product.Categories.Any(
                              cat => searchFor.Any(d => d.ID == cat.ID));

to get rid of the object comparison didn’t help. I’m stuck. How can I manage that?

Answer 1

If you’ve already got a list of categories, you can build a list of IDs outside the query:

var searchForIds = searchFor.Select(x => x.ID).ToList();
var query = context.Products
                   .Where(product => product.Categories
                                     .Any(cat => searchForIds.Contains(cat.ID)));

I don’t know that that will work, but it might. (Apologies for the indentation… it’s just to avoid scrolling.)