View Source Getting additional values

If you would like to get more than just the id from the record, you can use the extra_columns option.

The extra_columns option should be used with care because:

• The pg_notify function has a limit of 8000 characters and wasn't created to send full-records on updates.
• If many updates are done in quick succession to the same record, subscribers will need to process all of the old results before getting to the newest one.

Thus using extra_columns with columns that change often may not be what you want.

One use-case where using extra_columns may be particularly useful is if you want to receive updates about the deletion of a record and you need to know one of it's foreign keys. E.g. in a blog, if a Comment is deleted you might want to get the post_id to refresh any caches related to comments.

  # setup
  {EctoWatch,
   repo: MyApp.Repo,
   pub_sub: MyApp.PubSub,
   watchers: [
     # ...
     {Comment, :deleted, extra_columns: [:post_id]},
     # ...
   ]}

  # subscribing
  EctoWatch.subscribe({Comment, :deleted})

  # handling messages
  def handle_info({{Comment, :deleted}, %{id: id, post_id: post_id}}, socket) do
    Posts.refresh_cache(post_id)

    # ...