Hello all

Disclaimer: Since this is a work thing, I changed the name of the schemas. The structure is the same.

I’m currently working on a dashboard that shows me all the projects I have. Each project can have x tasks. I work with Ecto.Repo and have an association of has_many in the project schema to map to the tasks. The task schema has the field belongs_to project. That works well, I can add tasks and read them and everything.

On my dashboard I want to have an overview of all my active projects and the count of tasks. So basically a list with

• my first project (5 tasks)
• my second project (2 tasks)
  …

So far so good. My code looks like this

      Project
      |> select([p, t], %{
        id: p.id,
        title: p.title,
        state: p.state,
        task_count: count(t.id),
      })
      |> join(:left, [p], t in assoc(p, :tasks))
      |> where(^filter_by(filter))
      |> group_by([p], p.id)
      |> order_by(asc: :title)
      |> Repo.all()

The filter_by is a dynamic function that allows me to filter by term, state (and more to come). That works so I will skip the code for that. I can include it, if you feel it is necessary.

Now I want to paginate everything. So far, I’ve written a generic module to do that for me (because I will need it all the time for other stuff). So in the above code, I want to use that instead of Repo.all(). The call looks like this

[... all the query stuff from above except for the Repo.all() ]
|> Pagination.page(%{number: page_number, limit: page_limit}) 

And my module looks like this:

defmodule Infrastrucutre.Repo.Pagination do
  import Ecto.Query

def page(query, %{number: number, limit: page_limit}) do
    query
    |> limit([r], ^page_limit)
    |> offset([r], ^((number - 1) * page_limit))

    %{
      page: Repo.all(query),
      stats: %{
        count: count(query, %{page_limit: page_limit}),
        number: number
      }
    }
  end

  defp count(query, %{page_limit: page_limit}) do
    ceil(Repo.aggregate(query, :count) / page_limit)
  end
end

The alert reader will probably already see the problem. While this module worked fine for me so far, I am now for the first time trying to aggregate a query that already has an aggregation (= count(t.id)) in it. And that does not work at all because I can’t aggregate an aggregation. The excat problem being

** (Ecto.QueryError) cannot aggregate on query with group_by in query:

To solve this, I can have two queries in my repository, one for all my projects that I hand to the method count (I would have to make this public then) that gives me the count. And one for all my projects including the task count that I return in the pagination as “page”. Then I would prepare my pagination in my repository and return that.

Or I can have two methods in pagination. One with the limit and offset and one without where I can use Repo.all on and then get the length of all the entries without the limit and offset.

Both do not feel like clean code. So I am wondering if there is another way to do it that I haven’t thought of yet. What’s the most elixir way to do it?

Thanks for your time and input! Have a great day