Ash Elixir•2y ago

Calculations Using Relationships

I'm writing a simple chat app to try to learn Ash. I have models like below and I'd like to write an :unread_count calculation on the Channel. It would take the current user as the actor and find the associated member of the channel, use that :last_read_at time to filter down the messages associated with the channel to ones that has a :created_at after that time, and count them. I'm having a hard time figuring out how to do that based on the examples. What I think I want is something like:

  calculations do
    calculate :unread_count,
              :integer, expr(length(messages.channel_id == ^arg(:channel_id) && messages.created_at > current_member.last_read_at))
  end

  calculations do
    calculate :unread_count,
              :integer, expr(length(messages.channel_id == ^arg(:channel_id) && messages.created_at > current_member.last_read_at))
  end

Assuming :current_member is another calculation that exists to get the member associated with the current user. Apologies if I'm thinking about this in the completely wrong way and thanks in advance for the help! Models

defmodule App.Chat.Channel do
  relationships do
    many_to_many :members, App.Account.User do
      through App.Chat.ChannelMember
      source_attribute_on_join_resource :channel_id
      destination_attribute_on_join_resource :user_id
      writable? true
      filterable? true
    end

    has_many :messages, App.Chat.Message do
      sort created_at: :desc
      filterable? true
    end
  end
end

defmodule App.Chat.Channel do
  relationships do
    many_to_many :members, App.Account.User do
      through App.Chat.ChannelMember
      source_attribute_on_join_resource :channel_id
      destination_attribute_on_join_resource :user_id
      writable? true
      filterable? true
    end

    has_many :messages, App.Chat.Message do
      sort created_at: :desc
      filterable? true
    end
  end
end

defmodule App.Chat.ChannelMember do
  attributes do
    attribute :channel_last_read_at, :utc_datetime_usec
  end

  relationships do
    belongs_to :channel, App.Chat.Channel, primary_key?: true, allow_nil?: false
    belongs_to :user, App.Account.User, primary_key?: true, allow_nil?: false
  end
end

defmodule App.Chat.ChannelMember do
  attributes do
    attribute :channel_last_read_at, :utc_datetime_usec
  end

  relationships do
    belongs_to :channel, App.Chat.Channel, primary_key?: true, allow_nil?: false
    belongs_to :user, App.Account.User, primary_key?: true, allow_nil?: false
  end
end

defmodule App.Chat.Message do
  attributes do
    create_timestamp :created_at
  end

  relationships do
    belongs_to :channel, App.Chat.Channel do
      allow_nil? false
      attribute_writable? true
    end
  end
end

defmodule App.Chat.Message do
  attributes do
    create_timestamp :created_at
  end

  relationships do
    belongs_to :channel, App.Chat.Channel do
      allow_nil? false
      attribute_writable? true
    end
  end
end

4 Replies

ZachDaniel•2y ago

You're thinking about it in the right way, but this might be one that requires dipping down into SQL actually There are a few ways you can go about it You can do it in a separate query like so:

defmodule YourCalc do
  def calculate(records, _, _) do
    run_ecto_query
    |> group_by_record_id()
  end
end

defmodule YourCalc do
  def calculate(records, _, _) do
    run_ecto_query
    |> group_by_record_id()
  end
end

Theres a lot to unpack in that calculation 😆 What might be easier is adding calculation args Do you need to be able to filter/sort on this value?

bahalperinOP•2y ago

Thanks for the quick response! Nope, was just going to display it next to the name of the channel, like slack. When you say "adding calculation args" do you mean like having multiple separate calculations that take arguments? Like getting all the unread messages given a timestamp? And then do more of the work in separate steps outside of one calculation? Like load current channel member -> get unread messages given their last read timestamp?

ZachDaniel•2y ago

In that case, I'd suggest using a module-based calculation 🙂 You can run an ecto query or any other thing to get the data you want that way

bahalperinOP•2y ago

Just making sure I'm looking at the right thing, this is what you're talking about? https://ash-hq.org/docs/guides/ash/latest/topics/calculations#module-calculations. Think I missed those in the docs before. Thank you!!

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