Ecto Filter

filter.ex

defmodule Filters do
  import Ecto.{Query}, warn: false

  defmacro __using__(_opts) do
    quote do
      defp apply_filters(original_query, params) do
        filters = create_filters(params)

        query =
          try do
            Enum.reduce(filters, original_query, &filter/2)
          rescue
            _ -> original_query
          end

        maybe_apply_defaults(query, original_query, filters)
      end

      defp maybe_apply_defaults(query, original_query, filters) when query == original_query,
        do: Enum.reduce(filters, query, &defaults/2)

      defp maybe_apply_defaults(query, _, _), do: query

      defp condition_bools(filters) do
        Enum.map(filters, fn
          {key, "true"} -> {key, true}
          {key, "false"} -> {key, false}
          {key, value} -> {key, value}
        end)
      end

      defp filter_parameters(filters) do
        Enum.filter(filters, fn
          {_, nil} -> false
          {_, ""} -> false
          {key, value} -> true
          _ -> false
        end)
      end

      defp convert_string_keys(filters) do
        Enum.map(filters, fn
          {key, value} when is_binary(key) -> {String.to_existing_atom(key), value}
          {key, value} -> {key, value}
        end)
      end

      defp create_filters(%{"q" => parameters}) do
        parameters
        |> filter_parameters()
        |> condition_bools()
        |> convert_string_keys()
      end

      defp create_filters(_), do: []

      defp defaults({key, value}, query) do
        from(
          [c] in query,
          where: field(c, ^key) == ^value
        )
      end

      defp defaults(_, query), do: query
    end
  end
end