Aggregates with filter arguments

Ash Forum Ash Questions / Help
,
1

In my system, I have the following resources:

defmodule Core.Feedbacks.Student do
  relationships do
    alias Core.Feedbacks

    has_many :feedbacks, Feedbacks.Feedback

    many_to_many :classes, Feedbacks.Class do
      through Feedbacks.StudentClass
      join_relationship :student_classes

      source_attribute_on_join_resource :student_id
      destination_attribute_on_join_resource :class_id
    end
  end
end

defmodule Core.Feedbacks.Class do
  relationships do
    alias Core.Feedbacks

    has_many :feedbacks, Feedbacks.Feedback

    many_to_many :students, Feedbacks.Student do
      through Feedbacks.StudentClass

      source_attribute_on_join_resource :class_id
      destination_attribute_on_join_resource :student_id
    end
  end
end

defmodule Core.Feedbacks.StudentClass do
  relationships do
    alias Core.Feedbacks.{Student, Class}

    belongs_to :student, Student do
      primary_key? true
      allow_nil? false
      attribute_writable? true
    end

    belongs_to :class, Class do
      primary_key? true
      allow_nil? false
      attribute_writable? true
    end
  end
end

defmodule Core.Feedbacks.Feedback do
  relationships do
    alias Core.Feedbacks

    belongs_to :student, Feedbacks.Student do
      allow_nil? false
      attribute_writable? true
    end

    belongs_to :class, Feedbacks.Class do
      allow_nil? false
      attribute_writable? true
    end
  end
end

As you can see, the Student resource has a many_to_many relationship with the Class resource and both resources have a has_many relationship with the Feedback resource.

I wan´t to create an aggregate that will tell me how many feedbacks a student has inside a class.

If I just wanted all the student feedbacks, an aggregate like this in my Student resource would work just fine:

count :total_feedbacks, :feedbacks

But I want to count all the feedbacks that are from that student and from an specific class, meaning that I would to, somehow, pass an :class_id argument to inside that count aggregate filter expression.

I’m not sure how to solve that using aggregates

2

You can’t do that with standard aggregates, but you can do that with the new(-ish) inline aggregates syntax.

calculate :total_feedbacks, :integer, expr(
  count(feedbacks, query: [filter: expr(class_id == ^arg(:class_id))])
) do
  argument :class_id, :uuid, allow_nil?: false
end
3

As always, worked like a charm! :smile:

4

Hey @zachdaniel, I have another case where I’m not 100% sure how to solve it.

The calculations with inline aggregates worked great, but what if I want to aggregate something from one of the relationships?

For example.

Let’s say that the Class resource has a has_many relationship with another resource called Lessons.

I want to have a calculation in my Student resource that would give me number of lessons inside that class that a student is in.

What I tried to do was to first create an aggregate in the Class resource like this:

  aggregates do
    count :total_lessons, :lessons
  end

And then, inside the Student resource, I tried to create a calculation using that:

    calculate :total_lessons_in_class,
              :integer,
              expr(first(classes, query: [filter: expr(class_id == ^arg(:class_id))]).total_lessons) do
      argument :class_id, :string, allow_nil?: false
    end

But that doesn’t work.

I also tried adding a calculation that would return the current class and then I would be able to get the total_lessons field directly from it:

  calculate :current_class, :struct, expr(first(classes, query: [filter: expr(id == ^arg(:class_id))])) do
      constraints [instance_of: Core.Feedbacks.Class]

      argument :class_id, :string, allow_nil?: false
    end

But using the first aggregate doesn´t seem to be working, not sure if I’m doing something wrong with it.

5

So, what I’m about to show you wont’ work, but what you would want is this:

    calculate :total_lessons_in_class,
              :integer,
              expr(first(classes, query: [filter: expr(class_id == ^arg(:class_id))], field: :total_lessons)) do
      argument :class_id, :string, allow_nil?: false
    end

However, you can’t use an aggregate as the field of another aggregate. However, you can do this:

    calculate :total_lessons_in_class,
              :integer,
              expr(count(classes.lessons, query: [filter: expr(class_id == ^arg(:class_id))])) do
      argument :class_id, :string, allow_nil?: false
    end

Assuming that lesson has class_id on it.

6

Yep, this worked great, thanks again!