TIL: Ecto supports a query inside another query
If you read the Ecto.Query documentation, one of the first sections explains how Ecto queries are composable. Meaning, we can extend a query after creating it. Like so:
query = from e in Event, where: e.category == ^event.category case event.host do host -> from e in query, where: e.host == ^host _ -> query end
This was a feature that I learned a while after starting Elixir (apparently I didn’t read the docs well enough). And it is totally awesome. But today I learned Ecto also supports nested queries!
last_event = from e in Event, distinct: e.id, order_by: [desc: e.inserted_at] query = from a in Attendee, preload: [events: ^last_event]
Here, we are referencing a query (
last_event) from within another query. How exciting! It is also important to mention
that Ecto 2.0 supports subqueries in the
join fields (see example below). This makes queries even more malleable and powerful.
Hope you enjoy it as much as I do!
query = from e in Event, select: e q = from e in subquery(query), select: e.summary