How to connect Entity to Entity Freamework Core 2.0 Preview?

I want to create one method with which I can add / update an entity using Entity Framework 2.0-Preview.
I am using an insert update pattern.
For more information: https://msdn.microsoft.com/en-us/data/jj592676.aspx (last example)

Below is the code of the method:

    public string AttachEntity(Book book)
    {
        _context.Entry(book).State = (book.Id == 0)
            ? Microsoft.EntityFrameworkCore.EntityState.Added
            : Microsoft.EntityFrameworkCore.EntityState.Modified;
        string msg = $"Book details {_context.Entry(book).State} Successfully";

        _context.Book.Attach(book);
        _context.SaveChanges();
        return msg;
    }

      

        
        
        
      

    

and got below error:

InvalidOperationException: The Id property for the entity type "Book" has a temporary value when trying to change the state of the object to "No change". Either set a constant value or make sure the database is configured to generate values ​​for this property.

However, an identifier is automatically generated for the books table.

and below urls for this action:

[To add] Home / AttachEntity? Id = 1 & title = 4th book & Description = New book added by attach & Price = 120 & AuthorId = 2
[To update] Home / AttachEntity? title = 4th book & Description = New book added by attach & Price = 120 & AuthorId = 2