Add separate module for pagination of queries

lib/mobilizon/page.ex

@@ -0,0 +1,48 @@
+defmodule Mobilizon.Page do
+  @moduledoc """
+  Module for pagination of queries.
+  """
+
+  import Ecto.Query, warn: false
+
+  alias Mobilizon.Repo
+
+  defstruct [
+    :total,
+    :elements
+  ]
+
+  @type t :: %__MODULE__{
+          total: integer,
+          elements: struct
+        }
+
+  @doc """
+  Returns a Page struct for a query.
+  """
+  @spec build_page(Ecto.Query.t(), integer | nil, integer | nil) :: t
+  def build_page(query, page, limit) do
+    [total, elements] =
+      [
+        fn -> Repo.aggregate(query, :count, :id) end,
+        fn -> Repo.all(paginate(query, page, limit)) end
+      ]
+      |> Enum.map(&Task.async/1)
+      |> Enum.map(&Task.await/1)
+
+    %__MODULE__{total: total, elements: elements}
+  end
+
+  @doc """
+  Add limit and offset to the query.
+  """
+  @spec paginate(Ecto.Query.t(), integer | nil, integer | nil) :: Ecto.Query.t()
+  def paginate(query, page \\ 1, size \\ 10)
+
+  def paginate(query, page, _size) when is_nil(page), do: paginate(query)
+  def paginate(query, page, size) when is_nil(size), do: paginate(query, page)
+
+  def paginate(query, page, size) do
+    from(query, limit: ^size, offset: ^((page - 1) * size))
+  end
+end