Ash Elixir•2y ago

How To: Atomic update with a where clause

I'm trying to implement this workflow: Update a timestamp atomically on an existing record if the new timestamp is greater than the old one, and the old one hasn't changed while im updating it(optimistic update). I know atomics just dropped on Ash core so wondering if this could be achieved with it. This is the ash action in question:

    update :mark_as_paid do
      argument :invoice_event_ts, :utc_datetime_usec, allow_nil?: false

      change set_attribute(:invoice_status, :paid)
      change atomic_update(:last_invoice_event_ts, arg(:invoice_event_ts))
    end

    update :mark_as_paid do
      argument :invoice_event_ts, :utc_datetime_usec, allow_nil?: false

      change set_attribute(:invoice_status, :paid)
      change atomic_update(:last_invoice_event_ts, arg(:invoice_event_ts))
    end

5 Replies

gordonelielOP•2y ago

Basically want to do something like this:

update :mark_as_unpaid do
  argument :invoice_event_ts, :utc_datetime_usec, allow_nil?: false

  change set_attribute(:invoice_status, :unpaid)
  change atomic_update(:last_invoice_event_ts, arg(:invoice_event_ts)), where: expr(last_invoice_event_ts < ^arg(:invoice_event_ts)
end

update :mark_as_unpaid do
  argument :invoice_event_ts, :utc_datetime_usec, allow_nil?: false

  change set_attribute(:invoice_status, :unpaid)
  change atomic_update(:last_invoice_event_ts, arg(:invoice_event_ts)), where: expr(last_invoice_event_ts < ^arg(:invoice_event_ts)
end

ZachDaniel•2y ago

change atomic_update(:last_invoice_event_ts, expr(
  if last_invoice_event_ts < ^arg(:invoice_event_ds) do
    ^arg(:invoice_event_ds)
  else
    invoice_event_ds
  end
))

change atomic_update(:last_invoice_event_ts, expr(
  if last_invoice_event_ts < ^arg(:invoice_event_ds) do
    ^arg(:invoice_event_ds)
  else
    invoice_event_ds
  end
))

That should do it 🙂

gordonelielOP•2y ago

That works @Zach Daniel You're the best! How would you combine this with another attribute? Like say I want the invoice_status attribute to only update if the atomic last_invoice_event_ts is updated? Can that even work? Do I have to dip into a custom action / change for that? Something like so:

      change atomic_update(
               :invoice_status,
               expr(
                 cond do
                   # Not sure if you can get another field here
                   is_nil(last_invoice_event_ts) ->
                     ^arg(:invoice_event_ts)

                   last_invoice_event_ts < ^arg(:invoice_event_ts) ->
                     :unpaid

                   true ->
                     invoice_status
                 end
               )
             )

      change atomic_update(
               :last_invoice_event_ts,
               expr(
                 cond do
                   is_nil(last_invoice_event_ts) ->
                     ^arg(:invoice_event_ts)

                   last_invoice_event_ts < ^arg(:invoice_event_ts) ->
                     ^arg(:invoice_event_ts)

                   true ->
                     last_invoice_event_ts
                 end
               )
             )

      change atomic_update(
               :invoice_status,
               expr(
                 cond do
                   # Not sure if you can get another field here
                   is_nil(last_invoice_event_ts) ->
                     ^arg(:invoice_event_ts)

                   last_invoice_event_ts < ^arg(:invoice_event_ts) ->
                     :unpaid

                   true ->
                     invoice_status
                 end
               )
             )

      change atomic_update(
               :last_invoice_event_ts,
               expr(
                 cond do
                   is_nil(last_invoice_event_ts) ->
                     ^arg(:invoice_event_ts)

                   last_invoice_event_ts < ^arg(:invoice_event_ts) ->
                     ^arg(:invoice_event_ts)

                   true ->
                     last_invoice_event_ts
                 end
               )
             )

ZachDaniel•2y ago

You can get another field 🙂 You can't get the "new value", but you can get the old value and the argument

gordonelielOP•2y ago

Thanks that works! For posterity, This is my change module updating two values atomically:

  def change(changeset, [next_status: next_status], _context) do
    changeset
    |> Changeset.atomic_update(
      :invoice_status,
      expr(
        cond do
          is_nil(last_invoice_event_ts) ->
            ^next_status

          last_invoice_event_ts < ^arg(:invoice_event_ts) ->
            ^next_status

          true ->
            invoice_status
        end
      )
    )
    |> Changeset.atomic_update(
      :last_invoice_event_ts,
      expr(
        cond do
          is_nil(last_invoice_event_ts) ->
            ^arg(:invoice_event_ts)

          last_invoice_event_ts < ^arg(:invoice_event_ts) ->
            ^arg(:invoice_event_ts)

          true ->
            last_invoice_event_ts
        end
      )
    )
  end

  def change(changeset, [next_status: next_status], _context) do
    changeset
    |> Changeset.atomic_update(
      :invoice_status,
      expr(
        cond do
          is_nil(last_invoice_event_ts) ->
            ^next_status

          last_invoice_event_ts < ^arg(:invoice_event_ts) ->
            ^next_status

          true ->
            invoice_status
        end
      )
    )
    |> Changeset.atomic_update(
      :last_invoice_event_ts,
      expr(
        cond do
          is_nil(last_invoice_event_ts) ->
            ^arg(:invoice_event_ts)

          last_invoice_event_ts < ^arg(:invoice_event_ts) ->
            ^arg(:invoice_event_ts)

          true ->
            last_invoice_event_ts
        end
      )
    )
  end

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How To: Atomic update with a where clause

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