Entity Framework : updating one-many relationships for an entity's child collection

Question

Consider an application dealing with houses and pictures. An entity named House has a 1:n relationship to HousePicture.

The application allows the user to create and edit a House, and add/remove HousePictures.

[HttpPost]
public ActionMethod Edit(House h)
{
   //get an attached entity from the DB first.
   House original = db.Houses.SingleOrDefault(x=>x.ID==h.ID);

   UpdateModel(original);

   //collect all the uploaded pictures.
   original.HousePictures = GatherPicturesFromForm();


   db.SaveChanges();
   // the changes for the House are saved, 
   //but child collection Pictures are not.
}

How do you go about updating - adding new & deleting - the child collection when recreating the child collection from scratch?

• Add() or Attach() for each child in the collection?
• In what sequence do you need to Add or Attach the parent entity vs. the child collection?
• How to go about detecting the children to remove? Is this a feature of EF4 where deletes happen automatically by the framework, or does the developer need to write this logic?

• When adding more than one HousePicture, its ID == 0. The entity has the primary key in SQL Server has an auto-assigned PK of int IDENTITY(1,1). This becomes a problem because EF4 thinks that 2+ child have the same ID.

• What are your recommendations on saving child collections using Entity Framework 4?

• Any other suggestions on making the persistence of 1:n collections easier when updating/adding/deleting?

Answer 1

In EF just add new objects to a collection and they'll automatically be persisted to the database with the correct foreign key values.

original.HousePictures.AddRange(GatherPicturesFromForm());