How to Replace child collections in Entity Framework?

ORMs are nice. Usually. That is until you hit a mostly unhelpful exception like “An object with the same key already exists in the ObjectStateManager”. I ran into this problem by trying to do a complete replacement of an entity’s child entity collection (i.e. a Products’s IQueryable<Ingredient>). To do this, you actually need to remove all of the entities from the IQueryable that need to be removed, and then add the new entities. After some frustration, I created the following extension to help me out.

public static void ReplaceEntityCollection<T, K>(this ICollection<T> startingCollection, ICollection<T> newCollection,
 Func<T, K> selector )
{
 var newItemCompareValues = newCollection.Select(selector);
 var existingItemCompareValues = startingCollection.Select(selector);

 var alternativeItemsToRemove = startingCollection.Where(at => !newItemCompareValues.Contains(selector(at)));
 var alternativeItemsToAdd = newCollection.Where(at => !existingItemCompareValues.Contains(selector(at)));

 foreach (var alternativeItemToRemove in alternativeItemsToRemove)
 {
 startingCollection.Remove(alternativeItemToRemove);
 }

 foreach (var alternativeItemToAdd in alternativeItemsToAdd)
 {
 startingCollection.Add(alternativeItemToAdd);
 }
}

Now, I can do the following:

myProduct.Ingredients.ReplaceEntityCollection(myNewIngredients, ((Ingredient i) => i.Id));

This seems to have resolved that issue!