Ash Elixir•2y ago

adding for update to a read action

I have an after_action block that performs a select using a read action. I'd like to turn that select into a "SELECT FOR UPDATE" - is that possible?

22 Replies

rohanOP•2y ago

Nm! The answer was to use modify_query on the read transaction and then use Ecto.Query.lock("FOR UPDATE")

ZachDaniel•2y ago

You can use Ash.Query.lock(query, :for_update)

rohanOP•2y ago

sorry I might be being slow here, but how do I use this within a read action block eg.

    read :get_processed_by_index do
      # NOTE(rohan): We need FOR UPDATE so we are sure that it's not about to be marked as processed after we read it
      modify_query {Scribble.Scribe.AudioFile, :add_for_no_key_update, []}
      transaction? true

    
    end

    read :get_processed_by_index do
      # NOTE(rohan): We need FOR UPDATE so we are sure that it's not about to be marked as processed after we read it
      modify_query {Scribble.Scribe.AudioFile, :add_for_no_key_update, []}
      transaction? true

    
    end

ZachDaniel•2y ago

You can use a preparation

read :get_processed_by_index do
  # NOTE(rohan): We need FOR UPDATE so we are sure that it's not about to be marked as processed after we read it
  # modify_query {Scribble.Scribe.AudioFile, :add_for_no_key_update, []}
  prepare fn query, _ -> Ash.Query.lock(query, :for_update) end
  transaction? true
end

read :get_processed_by_index do
  # NOTE(rohan): We need FOR UPDATE so we are sure that it's not about to be marked as processed after we read it
  # modify_query {Scribble.Scribe.AudioFile, :add_for_no_key_update, []}
  prepare fn query, _ -> Ash.Query.lock(query, :for_update) end
  transaction? true
end

preparations are just like changes from create/update/destroy actions you can attach lifecycle hooks and things like that there

rohanOP•2y ago

got it!

ZachDaniel•2y ago

question: do you have an update action that youare passing this to? like get_processed_by_index -> update_it()? the thing you're doing isn't actually going to do what you want TBH

rohanOP•2y ago

not quite - it is part of an action that's going to do an update, but to a different row

ZachDaniel•2y ago

Ah, okay the main thing is that if you just call this action on its own, the transaction? true will end but if its called inside another transaction then it will do what you want

rohanOP•2y ago

yup - this is in an after action block

ZachDaniel•2y ago

👍

rohanOP•2y ago

i probably didn't need the transaction? true but it felt right to put it there 🙂 ash doesn't currently support FOR NO KEY UPDATE right? is there a way to pass that in or would that require an ash PR

ZachDaniel•2y ago

IIRC if you pass a string it will use it directly in the data layer

rohanOP•2y ago

oh perfect!

ZachDaniel•2y ago

you can do this to protect yourself in the future:

prepare fn query, _ -> 
  Ash.Query.before_action(query, fn query -> 
    if Ash.DataLayer.in_transaction?(query.resource) do
      Ash.Query.add_error(query, "must be in a transaction")
    else
      query
    end
  end)
end

prepare fn query, _ -> 
  Ash.Query.before_action(query, fn query -> 
    if Ash.DataLayer.in_transaction?(query.resource) do
      Ash.Query.add_error(query, "must be in a transaction")
    else
      query
    end
  end)
end

rohanOP•2y ago

is there a similar thing for change get_and_lock("FOR NO KEY UPDATE")

ZachDaniel•2y ago

I believe so, yeah and then I'd remove the transaction? true just adds some protection that an action meant to set up something specific isn't called in a context where its not actually helping

rohanOP•2y ago

got it! thank you!

ZachDaniel•2y ago

👍 my pleasure

rohanOP•2y ago

    prepare fn query, _ -> Ash.Query.lock(query, "FOR NO KEY UPDATE") end

    prepare fn query, _ -> Ash.Query.lock(query, "FOR NO KEY UPDATE") end

results in:

** (Ash.Error.Invalid) Input Invalid

* Data layer for Scribble.Scribe.AudioFile does not support lock: "FOR NO KEY UPDATE"
    (ash 2.10.1) lib/ash/api/api.ex:2123: Ash.Api.unwrap_or_raise!/3

** (Ash.Error.Invalid) Input Invalid

* Data layer for Scribble.Scribe.AudioFile does not support lock: "FOR NO KEY UPDATE"
    (ash 2.10.1) lib/ash/api/api.ex:2123: Ash.Api.unwrap_or_raise!/3

(i'm using postgres)

ZachDaniel•2y ago

Hmm....what is your ash_postgres version?

iex(1)> AshPostgres.DataLayer.can?(nil, {:lock, "FOR NO KEY UPDATE"})
true

iex(1)> AshPostgres.DataLayer.can?(nil, {:lock, "FOR NO KEY UPDATE"})
true

rohanOP•2y ago

i was pointing at the main branch - but forcing an update now that was it - ty!

ZachDaniel•2y ago

👍

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adding for update to a read action

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