chuckconway/collection-comparer

Conway Collection Comparer

The Collection Comparer compares two collections and exposes an opportunity to execute code depending on if the item is in the source (left) collection, the comparing (right) collection, or both collections.

For example, I have two collections. One collection is from the database, a second from the UI. In the UI, the user removes some items and adds some new items. When the user saves the changes, we need to reconcile the differences.

One possibility is removing all the items and add them anew. The problem is if you have foreign keys, it quickly becomes challenging. An alternative is discovering the differences and update the database accordingly. This is where the Collection Comparer comes in.

An implementation might look like this:

private static void MergeAlbums(IEnumerable<int> existing, IEnumerable<int> newAlbums, PhotoSchema photo)
{
    existing.CompareTo(newAlbums, (s, d) => s == d)
        .OnlyInSourceCollection(s =>
        {
           // if only in the database collection, then it's been removed. 
            var album = photo.Albums.SingleOrDefault(x => x.AlbumId == s);
            photo.Albums.Remove(album);
        })
        .OnlyInComparingCollection(s =>
        {
            // if it's only in the comparing collection, then it's new
            // and needs to be added to the database.
            photo.Albums.Add(new PhotoAlbumSchema
            {
                AlbumId = s,
                PhotoId = photo.PhotoId
            });
        })
        .Process();
}

Another example:

var source = new []{1, 3, 4, 6};
var collection = new[] {1, 2, 3, 4, 5};

source.CompareTo(collection, (s, d) => s == d)
    .OnlyInSourceCollection(s=> {/* do something */})
    .OnlyInComparingCollection(s=>{/* do something */})
    .InBoth(s=> {/*do something*/})
    .Process();