Simple version of parsing order_by in query structs

lib/paginator/ecto/order_by.ex

@@ -0,0 +1,32 @@
+defmodule Paginator.Ecto.OrderBy do
+  @moduledoc false
+
+  # import Ecto.Query
+
+  def infer_order_by(queryable) do
+    queryable
+    |> get_order_by_expressions()
+    |> make_cursor_field_list()
+  end
+
+  defp get_order_by_expressions(queryable) do
+    queryable.order_bys
+    |> Enum.reduce([], fn x, acc -> x.expr ++ acc end)
+  end
+
+  defp make_cursor_field_list(expressions) do
+    expressions
+    |> Enum.map(fn {key, value} ->
+      # gets the field name atom from the query struct
+
+      case value do
+        # https://github.com/elixir-ecto/ecto/blob/1533413/lib/ecto/query/builder/order_by.ex#L121
+        {{:., [], [{:&, [], [0]}, field]}, [], []} ->
+          {field, key}
+
+        _ ->
+          raise "Unsupported `order_by` syntax, could not infer the cursor fields. Please supply `cursor_fields` manually."
+      end
+    end)
+  end
+end

test/paginator/order_by_test.exs

@@ -0,0 +1,48 @@
+defmodule Paginator.OrderByTest do
+  use ExUnit.Case, async: true
+
+  alias Paginator.Ecto.OrderBy
+  import Ecto.Query
+
+  defp assert_result(result, list) do
+    assert(result == list)
+  end
+
+  describe "OrderBy.infer_order_by/1" do
+    test "parses single order_by with default direction" do
+      from(p in "payments",
+        order_by: p.charged_at
+      )
+      |> OrderBy.infer_order_by()
+      |> assert_result(charged_at: :asc)
+    end
+
+    test "parses multiple order_bys with default direction" do
+      from(p in "payments",
+        order_by: [p.amount, p.charged_at]
+      )
+      |> OrderBy.infer_order_by()
+      |> assert_result(amount: :asc, charged_at: :asc)
+    end
+
+    test "parses multiple seperate order_bys with a given direction" do
+      from(p in "payments",
+        order_by: [desc: p.charged_at],
+        order_by: [asc: p.amount]
+      )
+      |> OrderBy.infer_order_by()
+      |> assert_result(amount: :asc, charged_at: :desc)
+    end
+
+    test "rejects order_by with a fragment" do
+      assert_raise RuntimeError,
+                   "Unsupported `order_by` syntax, could not infer the cursor fields. Please supply `cursor_fields` manually.",
+                   fn ->
+                     from(p in "payments",
+                       order_by: fragment("amount")
+                     )
+                     |> OrderBy.infer_order_by()
+                   end
+    end
+  end
+end