Best approach for my EF 'DataBase'

I've been a few weeks working on a web project, amd mostly thinking how would I implement data layer. I chosed Entity Framework 4.1, code first model.

So, among lot of other entities , think of PLAYER who has N CHARACTER, that can be in 0..1 GUILD

public class Player
{
    public int Id { get; set; }
    public string Login { get; set; }
    public string Password { get; set; }
    public bool IsLogged { get; set; }
    public DateTime LastActivity { get; set; }
    public DateTime LastLogin { get; set; }
    public DateTime LastLogout { get; set; }
    public string DisplayName { get; set; }
    public string DefaultImage { get; set; }

    public virtual Board Board { get; set; }
    public virtual PlayerData PlayerData { get; set; }
    public virtual ICollection<Character> Characters { get; set; }
}
public class Guild
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string DefaultImage { get; set; }

    public virtual ICollection<Character> Characters { get; set; }
}
public class Character
{
    public int Id { get; set; }
    public string Name { get; set; }

    public virtual Player Player { get; set; }
    public virtual Guild Guild { get; set; }
    public virtual GuildRank GuildRank { get; set; }
    public virtual Game Game { get; set; }
}

As you can see, there a lot more entities and relationships, but this will work.

Well it does not, this code:

Character character = mod.Characters.Where(c => c.Player == player).FirstOrDefault();

Throws an exception:

Unable开发者_高级运维 to create a constant value of type 'DataObjects.Player'. Only primitive types ('such as Int32, String, and Guid') are supported in this context.

I don't understand why.

I also tried with using [Key] and [ForeingKey] attributes, but I can't find them! :S (though the where in System.Data.Entity.dll, but the don't).

So after so many errors, I started to think maybe I got the whole thing wrong...

Any ideas on how to fix the error, or to go in other direction?

Thanks in advance.

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This is stupidity in EF. You cannot compare Player directly. You must compare Ids so your query can be rewritten to:

int playerId = player.Id;
Character character = mod.Characters.FirstOrDefault(c => c.Player.Id == playerId);