实体框架插入数据库中已存在的实体

我使用的是EF 6 Code First MVC plus标识

我自定义了Identity framework中定义的标准ApplicationUser类,当我尝试通过网页注册新用户时,EF尝试插入数据库中已经存在的实体。

//some code above here

using (ApplicationDbContext db = new ApplicationDbContext())
{
    // I load the City object form db based on the model passed to current method
    City city = db.Countries.Include("States.Cities")
        .First(c => c.Code == model.CountryCode)
        .States.First(s => s.Id == model.StateId)
        .Cities.First(ci => ci.Name == model.CityName);

    // I create a new Employee and a new Company and Company has an Address 
    // referring to the City I loaded from db
    Employee employee = new Employee
    {
        FirstName = model.FirstName,
        LastName = model.LastName,
        Company = new Company
        {
            Name = model.CompanyName,
            OfficePhone = model.OfficePhone,
            Address = new Address { City = city, Street = model.Street, ZipCode = model.ZipCode }
        }
    };

    // This is part of Identity framework code
    var user = new ApplicationUser
    {
        UserName = model.Email,
        Email = model.Email,
        // my custom code: I assign employee to the standard ApplicationUser class
        UserProfile = employee
    };

    // This is going to save the new user to database, 
    // but it tries to insert a new Country object into db. 
    // Note that City class has a property of type State 
    // and State class has a property type of Country 
    var result = await UserManager.CreateAsync(user, model.Password);

    // more code below

根据我得到的错误,听起来EF正在将Country实体插入db。我猜当EF向db添加地址时会发生这种情况。在我的代码中,Address是一个新对象,但City、State和Country已经存在了,您可以看到我一开始是从db加载City的。因此,我尝试使用以下代码,但没有任何帮助:

db.Entry(employee.Company.Address.City).State = EntityState.Unchanged;

很抱歉,代码太长了,但我尝试将其最小化。

这是我的模型:

public class Country
{
    string Code;
    string Name;
    List<State> States;
}

public class State
{
    int Id;
    string Name;
    List<City> Cities;
}

public class City
{
    int Id;
    string Name;
    State State;
}

public class Address
{
    string Street;
    string ZipCode;
    City City;
}

public class Company
{
    string Name;
    string OfficePhone;
    Address Address;
}