Elixir module to use for Ecto models. This macro will implement Rails-like finder functions for the given module.

finder_functions.ex

defmodule Common.FinderFunctions do
  @moduledoc """
  Certainly not a perfect replication of Rails' finder methods, but at least gives Rails developers
  some sense of home with Ecto. This does not reach into associations, nor is it huge-db friendly.

  ## Usage:
  0. Don't. Just learn Ecto.
  1. In your given module that implements Ecto's schema, let it
      `use Common.FinderFunctions, repo: MyApp.Repo`. This also assumes there
      is a changeset function on your model.
  2. Anywhere you're using the module, (eg User) you can now use the functions
      below; eg:
      `User.find(1)`
      `User.find_or_create_by(email: "[email protected]")`
      `User.where(token: nil)`
  """
  defmacro __using__(opts) do
    repo = Keyword.get(opts, :repo)

    quote bind_quoted: [repo: repo] do
      import Ecto.Query

      @doc """
        Find records by ID

        If given one ID, returns a struct
        If given a list of IDs, returns a list of structs.

        ## Examples

            iex> User.find(1)
            %MyApp.User{__meta__: #Ecto.Schema.Metadata<:loaded, "users">,
              email: "[email protected]",
              id: 1,
              password: "mysecretpassword",
              inserted_at: #Ecto.DateTime<2016-09-19 15:48:52>,
              updated_at: #Ecto.DateTime<2016-09-19 19:12:00>}

            iex> User.find([1, 2, 3])
            [%MyApp.User{__meta__: #Ecto.Schema.Metadata<:loaded, "users">,
              email: "[email protected]",
              id: 1,
              password: "mysecretpassword",
              inserted_at: #Ecto.DateTime<2016-09-19 15:48:52>,
              updated_at: #Ecto.DateTime<2016-09-19 19:12:00>}]

      """

      def find_by(params), do: unquote(repo).get_by(__MODULE__, params)

      def find(queryable, id), do: from row in queryable, where: [id: ^id]
      def find(ids) when is_list(ids), do: where(id: ids)
      def find(id), do: unquote(repo).get(__MODULE__, id)

      def count(queryable \\ __MODULE__) do
        queryable |> select([r], count(r.id)) |> unquote(repo).one
      end

      def all(queryable \\ __MODULE__), do: unquote(repo).all(queryable)

      def where(params) do
        params
        |> find_by_fields
        |> unquote(repo).all
      end

      def find_or_create_by(params) when is_list(params) do
        params
        |> Enum.into(%{})
        |> find_or_create_by
      end

      def find_or_create_by(params) do
        case find_by(params) do
          nil    -> create_by(params)
          record -> record
        end
      end

      def find_or_initialize_by(params) when is_list(params) do
        params
        |> Enum.into(%{})
        |> find_or_initialize_by
      end

      def find_or_initialize_by(params) do
        case find_by(params) do
          nil    -> initialize_by(params)
          record -> record
        end
      end

      def create_by(params) when is_list(params) do
        params
        |> Enum.into(%{})
        |> create_by
      end

      def create_by(params) do
        changeset = initialize_by(params)
        case changeset.valid? do
          false -> {:error, changeset}
          true  ->
            params
            |> initialize_by
            |> unquote(repo).insert
        end
      end

      def initialize_by(params) when is_list(params) do
        params
        |> Enum.into(%{})
        |> initialize_by
      end

      def initialize_by(params) do
        %{ __MODULE__.changeset(__MODULE__.__struct__, params) | action: :insert }
      end

      defp find_by_field(queryable, db_field, search_term) when is_list(search_term) do
        from row in queryable, where: field(row, ^db_field) in ^search_term
      end

      defp find_by_field(queryable, db_field, search_term) when is_nil(search_term) do
        from row in queryable, where: is_nil(field(row, ^db_field))
      end

      defp find_by_field(queryable, db_field, search_term) when is_bitstring(search_term) do
        db_field = String.to_existing_atom(db_field)
        from row in queryable, where: field(row, ^db_field) == ^search_term
      end

      defp find_by_field(queryable, db_field, search_term) do
        from row in queryable, where: field(row, ^db_field) == ^search_term
      end

      defp find_by_fields(params) do
        params
        |> Enum.into(%{})
        |> Enum.reduce(
            __MODULE__,
            fn({db_field, value}, query) -> find_by_field(query, db_field, value) end
          )
      end
    end
  end
end