How to properly use aggregates on a resource?

Aggregates are “per instance of this resource”.

I think something like this:

# in your resource
defmodule MyApp.Invoice do
  actions do
    read :not_settled do
      filter expr(not settled)
    end
  end
end

# in your domain
defmodule MyApp.MyDomain do
  resource MyApp.Invoice do
    read :get_not_settled_invoices, action: :not_settled
  end
end

# consumer
MyApp.MyDomain.get_not_settled_invoices() |> Ash.sum(:amount)

but I don’t know if thats the best way to do it? I mean you can also put this function in your domain or wherever you like.

MyApp.Invoice |> Ash.Query.filter(expr(not settled)) |> Ash.sum(:amount) works for me.

Generally you want to move these things to read actions though and not just freestyle it everywhere.

I think what you want is to put the aggregate into the parent resource and then you can just load the aggregate from the parent resource. Something has those invoices right?

Ash.aggregate(Resource, {:total_amount, :sum}) should be requiring that you specify a field to be summing, i.e Ash.aggregate(Resource, {:total_amount, :sum, field: :amount}).

Ash.aggregate is for aggregating information about a query/resource as a whole. i.e

Resource
|> Ash.Query.filter(not settled)
|> Ash.aggregate(Resource, {:total_amount, :sum, field: :amount}

will get you back something like this:
{:ok, %{total_amount: #Decimal<10.0>}}, and is the “sum of of all amounts that are not settled”.

The aggregates declared in a resource are different, they are more like a calculation. They are for summary information about related data.

# on a customer resource for example
sum :total_unsettled_invoices_amount, [:invoices], :total_amount do
  filter expr(not settled)
end

This allows you to say:

customer |> Ash.load!(:total_unsettled_invoices_amount)

which return something like

%Customer{total_unsettled_invoices_amount: #Decimal<10.0>}

That total_invoices_amount is “for the given customer(s), the sum of all related invoices, which are not settled”

Hopefully that helps!