When to use Include in EF? Not needed in projection?

Question

I have the following in Entity Framework Core:

public class Book {
  public Int32 Id { get; set; }
  public String Title { get; set; }
  public virtual Theme Theme { get; set; }
}

public class Theme {
  public Int32 Id { get; set; }
  public String Name { get; set; }
  public Byte[] Illustration { get; set; }
  public virtual ICollection<Ebook> Ebooks { get; set; }
}

And I have the following linq query:

List<BookModel> books = await context.Books.Select(x =>
  new BookModel {
    Id = x.Id,
    Name = x.Name,
    Theme = new ThemeModel {
      Id = x.Theme.Id,
      Name = x.Theme.Name
    }
   }).ToListAsync();

I didn't need to include the Theme to make this work, e.g:

List<BookModel> books = await context.Books.Include(x => x.Theme).Select(x => ...

When will I need to use Include in Entity Framework?

UPDATE

I added a column of type Byte[] Illustration in Theme. In my projection I am not including that column so will it be loaded if I use Include? Or is never loaded unless I have it in the projection?

Answer 1

In search for an official answer to your question from Microsoft's side, I found this quote from Diego Vega (part of the Entity Framework and .NET team) made at the aspnet/Announcements github repository:

A very common issue we see when looking at user LINQ queries is the use of Include() where it is unnecessary and cannot be honored. The typical pattern usually looks something like this:

var pids = context.Orders
    .Include(o => o.Product)
    .Where(o => o.Product.Name == "Baked Beans")
    .Select(o =>o.ProductId)
    .ToList();

One might assume that the Include operation here is required because of the reference to the Product navigation property in the Where and Select operations. However, in EF Core, these two things are orthogonal: Include controls which navigation properties are loaded in entities returned in the final results, and our LINQ translator can directly translate expressions involving navigation properties.