c# - Entity Framework: field of composite key cannot be nullable? -

i have model composite key - row key:

public class item {     [key, column(order = 0)]     public int userid { get; set; }     [key, column(order = 1)]     public datetime? date { get; set; } } 

running code below throws exception dbentityvalidationexception message: the date field required.:

var = new item { date = null, userid = 2 }; m_entities.items.add(it); m_entities.savechanges(); // throws exception 

(m_entities usual dbcontext descendant items defined dbset<item>) why date required if can null (declared datetime?) ? , how allow null valid value date?

answer raphael lead me search. here why not possible (answer cobsy):

what's wrong nullable columns in composite primary keys?

in short: null == null -> false

wierd. solution me add id column model.

btw: mysql allow me not define primary key, i'm allowed have such schema - ef complains not defining key :-(.