Updating a slightly complex entity in EntityFrameworkCore

I am trying out EntityFrameworkCore. I looked at the documentation, but couldn't find a way to easily update a complex entity that is related to another entity.

Here is a simple example. I have 2 classes - Company & Employee.

public class Company
{
    public int Id { get; set; }
    public string Name { get; set; }
}

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
    public Company Company { get; set; }
}

Company is a simple class, and Employee is only slightly complex, as it contains a property with reference to the Company class.

In my action method, which takes in the updated entity, I could first look up the existing entity by id, and then set each property on it before I call SaveChanges.

[HttpPut]
public IActionResult Update(int id, [FromBody]Employee updatedEmployee)
{
    if (updatedEmployee == null || updatedEmployee.Id != id)
        return BadRequest();

    var existingEmployee = _dbContext.Employees
                             .FirstOrDefault(m => m.Id == id);
    if (existingEmployee == null)
        return NotFound();

    existingEmployee.Name = updatedEmployee.Name;

    if (updatedEmployee.Company == null)
        existingEmployee.Company = null; //as this is not a PATCH            
    else
    {
        var existingCompany = _dbContext.Companies.FirstOrDefault(m =>
                                m.Id == updatedEmployee.Company.Id);
        existingEmployee.Company = existingCompany;
    }

    _dbContext.SaveChanges();

    return NoContent();
}

With this sample data, I make an HTTP PUT call on Employees/3.

{
    "id": 3,
    "name": "Road Runner",
    "company":
    {
        "id": 1
    }
}

And that works.

But, I hope to avoid having to set each property this way. Is there a way I could replace the existing entity with the new one, with a simple call such as this?

_dbContext.Entry(existingEmployee).Context.Update(updatedEmployee);

When I try this, it gives this error:

System.InvalidOperationException: The instance of entity type 'Employee' cannot be tracked because another instance of this type with the same key is already being tracked. When adding new entities, for most key types a unique tem porary key value will be created if no key is set (i.e. if the key property is assigned the default value for its t ype). If you are explicitly setting key values for new entities, ensure they do not collide with existing entities or temporary values generated for other new entities. When attaching existing entities, ensure that only one entity instance with a given key value is attached to the context.

I can avoid this error if I retrieve the existing entity without tracking it.

var existingEmployee = _dbContext.Employees.AsNoTracking()
                         .FirstOrDefault(m => m.Id == id);

And this works for simple entities, but if this entity has references to other entities, this causes an UPDATE statement for each of those referenced entities as well, which is not within the scope of the current entity update. The documentation for the Update method says that as well:

// Begins tracking the given entity, and any other reachable entities that are not already being tracked, in the Microsoft.EntityFrameworkCore.EntityState.Modified state such that they will be updated in the database when Microsoft.EntityFrameworkCore.DbContext.SaveChanges is called.

In this case, when I update the Employee entity, my Company entity changes from

{
  "id": 1,
  "name": "Acme Products"
}

to

{
  "id": 1,
  "name": null
}

How can I avoid the updates on the related entities?

UPDATE:

Based on the inputs in the comments and the accepted answer, this is what I ended up with:

Updated Employee class to include a property for CompanyId in addition to having a navigational property for Company. I don't like doing this as there are 2 ways in which the company id is contained within Employee, but this is what works best with EF.

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
    public int CompanyId { get; set; }
    public Company Company { get; set; }
}

And now my Update simply becomes:

[HttpPut]
public IActionResult Update(int id, [FromBody]Employee updatedEmployee)
{
    if (updatedEmployee == null || updatedEmployee.Id != id)
        return BadRequest();

    var existingEmployeeCount = _dbContext.Employees.Count(m => m.Id == id);
    if (existingEmployeeCount != 1)
        return NotFound();

    _dbContext.Update(updatedEmployee);

    _dbContext.SaveChanges();

    return NoContent();
}