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.

Answer 2

I also tried the second option it worked for me. I did like the parent->child relationship happening at an object level first and save to db. Maybe I should just remove all the relationships between the entities that EF generates and manually control this myself.

Answer 3

Entity Framework does not allow relationships that cross contexts.

If you place the reading of the course and connecting the course to the student within the same using statement, it would work.