// Это таблица 
 public class Account
    {
        //код пользователя
        [Key]
        public int Object_Id {get; set;}
        [Required]
        public virtual Company CompanyOwner { get; set; }
        public string Login { get; set; }
        public string Password { get; set; }
        public virtual Account UserOwner { get; set; }
        [Required]
        public virtual AccountGroup AGroup { get; set; } 
    }


  //   Это код

   MyAppContext db = new MyAppContext();

   Account currentAccount = db.Accounts.FirstOrDefault(x => x.Object_Id == AccountId);
   currentAccount.Password = NewPassword;
   try
            {
                db.SaveChanges();
            }
            catch (System.Data.Entity.Validation.DbEntityValidationException ex)
            {
              Вот сюда и вываливается ссылаясь на то, что не установлены поля: CompanyOwner и AccountGroup
            }

Account currentAccount = db.Accounts.Include("CompanyOwner").Include("AGroup").FirstOrDefault(x => x.Object_Id == AccountId);

db.Accounts.Include("CompanyOwner").Include("AGroup").FirstOrDefault(x => x.Object_Id == AccountId);

exec sp_executesql N'SELECT 
    [Limit1].[Object_Id] AS [Object_Id], 
    [Limit1].[Login] AS [Login], 
    [Limit1].[Password] AS [Password], 
    [Limit1].[Object_Id1] AS [Object_Id1], 
    [Limit1].[CompanyName] AS [CompanyName], 
    [Limit1].[Owner_Object_Id] AS [Owner_Object_Id], 
    [Limit1].[Object_Id2] AS [Object_Id2], 
    [Limit1].[GroupName] AS [GroupName], 
    [Limit1].[UserOwner_Object_Id] AS [UserOwner_Object_Id]
    FROM ( SELECT TOP (1) 
        [Extent1].[Object_Id] AS [Object_Id], 
        [Extent1].[Login] AS [Login], 
        [Extent1].[Password] AS [Password], 
        [Extent1].[UserOwner_Object_Id] AS [UserOwner_Object_Id], 
        [Extent2].[Object_Id] AS [Object_Id1], 
        [Extent2].[CompanyName] AS [CompanyName], 
        [Extent2].[Owner_Object_Id] AS [Owner_Object_Id], 
        [Extent3].[Object_Id] AS [Object_Id2], 
        [Extent3].[GroupName] AS [GroupName]
        FROM   [dbo].[Accounts] AS [Extent1]
        INNER JOIN [dbo].[Companies] AS [Extent2] ON [Extent1].[CompanyOwner_Object_Id] = [Extent2].[Object_Id]
        INNER JOIN [dbo].[AccountGroups] AS [Extent3] ON [Extent1].[AGroup_Object_Id] = [Extent3].[Object_Id]
        WHERE [Extent1].[Object_Id] = @p__linq__0
    )  AS [Limit1]',N'@p__linq__0 int',@p__linq__0=1
go

exec sp_executesql N'UPDATE [dbo].[Accounts]
SET [Password] = @0
WHERE ([Object_Id] = @1)
',N'@0 nvarchar(500),@1 int',@0=N'41890cd2ac71e06b5f2c9ad5ccc07b45',@1=1
go