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

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 from and 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


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