For updating database from disconnected entities.

EFHelpers.cs

public static class EFHelpers
{
	/// <summary>
	/// Helper function to update an EF model child collection from a view model posted from javascript.
	/// </summary>
	public static void MergeGraph<TModel, TUpdate, TId>(
	   ICollection<TModel> modelCollection,
	   ICollection<TUpdate> updateCollection,
	   DbSet<TModel> dbSet,
	   Func<TModel, TId> modelId,
	   Func<TUpdate, TId> updateId,
	   Action<TModel, TUpdate> updateCallback)
		where TModel : class, new()
	{
		var addedOptions = updateCollection.Where(o => modelCollection.All(d => !EqualityComparer<TId>.Default.Equals(modelId(d), updateId(o)))).ToList();
		var removedOptions = modelCollection.Where(o => updateCollection.All(d => !EqualityComparer<TId>.Default.Equals(modelId(o), updateId(d)))).ToList();
		var updatedOptions = updateCollection.Except(addedOptions).ToList();

		foreach (var added in addedOptions)
		{
			var option = new TModel();
			updateCallback(option, added);
			modelCollection.Add(option);
		}

		foreach (var removed in removedOptions)
		{
			dbSet.Remove(removed);
			modelCollection.Remove(removed);
		}

		foreach (var update in updatedOptions)
		{
			var original = modelCollection.FirstOrDefault(o => EqualityComparer<TId>.Default.Equals(modelId(o), updateId(update)));
			if (original != null)
			{
				updateCallback(original, update);
			}
		}
	}
}

Usage.cs

public ActionResult Save(Parent updated)
{
  var model = context.// load from database
  
  EFHelpers.MergeGraph(
    model.Children,   // the collection to update
    updated.Children, // the posted changes
    context.Children, // the DbSet for the collection entities
    c => c.ChildId,   // a selector for the ID or matching key property on the model entity
    c => c.ChildId,   // a selector for the ID or matching key property on the posted entity
    UpdateChild       // a function to update an individual child entity
    );
}

public void UpdateChild(Child model, Child updated)
{
  // update child logic
  model.SomeProp = updated.SomeProp;
}