Entity Framework: adding existing child POCO to new Parent POCO, creates new child in DB

Question

Im wanting to use Entity Framework POCO in a disconnected (from context) mode. In my scenario I\'m creating a new Parent object and want to attach an existing child object

Answer 1

I believe there are few ways of accomplishing this. You can specify that course entity is unchanged rather than added, along these lines:

ctx.Entry(course).State = EntityState.Unchanged;

Or instruct your context, that you are working with existing entity:

ctx.Courses.Attach(course);

More info here: http://msdn.microsoft.com/en-us/data/jj592676.aspx

EDIT

There are some running samples from my solution, I verified they work as expected. In all cases we have Publisher record in database with ID = 2 and Name = "Addison Wesley" (irrelevant to the example, but just for good measure).

Approach 1 - Setting Entity State

using (var context = new Context())
{
    var book = new Book();
    book.Name = "Service Design Patterns";
    book.Publisher = new Publisher() {Id = 2 }; // Only ID is required
    context.Entry(book.Publisher).State = EntityState.Unchanged;
    context.Books.Add(book);
    context.SaveChanges();
}

Approach 2 - Using Attach method

using (var context = new Context())
{
    var book = new Book();
    book.Name = "Service Design Patterns";                
    book.Publisher = new Publisher() { Id = 2 }; // Only ID is required
    context.Publishers.Attach(book.Publisher);
    context.Books.Add(book);
    context.SaveChanges();
}

Approach 3 - Setting Foreign Key value

using (var context = new Context())
{
     var book = new Book();
     book.Name = "Service Design Patterns";
     book.PublisherId = 2; 
     context.Books.Add(book);
     context.SaveChanges();
}

For this last approach to work I needed to add extra property PublisherId, it has to be named according to NavigationPropertyName + 'Id" convention to be picked up by EF auotmatically:

public int PublisherId { get; set; }
public Publisher Publisher { get; set; }

I am using here EF5 Code First, but it is very similar to POCO.