Running tasks in production with Elixir releases: rpc, eval, and remote

You’re right, and thanks for calling it out. I’ve updated the post and re-recorded that section of the video.

Everything was fine and dandy until I encountered this N+1:

def reprice_stale do
  repriced =
    stale_query()
    |> Repo.all()
    |> Enum.map(fn widget ->
      widget
      |> Ecto.Changeset.change(
        price: widget.price |> Decimal.mult(@discount) |> Decimal.round(2),
        price_reduced: true
      )
      |> Repo.update!()
    end)

  IO.puts("Reduced #{length(repriced)} stale widgets")
end

I really, really hope the author doesn’t put such code in production because it’s essentially a single UPDATE in the database. No mapping, no individual updates.

def reprice_stale do
  {repriced_count, _} =
    stale_query()
    |> update([w], 
      set: [
        price: fragment("round(? * ?, 2)", w.price, @discount), 
        price_reduced: true
      ]
    )  
    |> Repo.update_all([])

  IO.puts("Reduced #{length(repriced_count)} stale widgets")
end

P.S. Of course, the above implies the author has a NUMERIC column for the price.