defmodule EctoMerge do
  @moduledoc """
  This module holds helper funcions for recursively
  merging ecto records ands it's associations.
  """

  import __MODULE__.Validator, only: [validate_mergeable!: 1]
  import __MODULE__.AssociationMerger, only: [merge_assocs: 4]

  alias __MODULE__.Utils
  alias Ecto.{Changeset, Multi}

  @doc "Merges a list of records and it's associations recursively"
  def merge!(repo, records) do
    validate_mergeable!(records)

    {changeset, records_to_delete} =
      merge_into_changeset(repo, records)

    {:ok, %{result: result}} =
      Multi.new
      |> Multi.insert(:result, changeset)
      |> delete(records_to_delete ++ records)
      |> repo.transaction

    result
  end

  defp merge_into_changeset(repo, records) do
    schema = get_schema(records)

    assocs = schema.__schema__(:associations)
    assocs_info = Enum.map(assocs, &schema.__schema__(:association, &1))

    base_changeset =
      schema
      |> struct()
      |> schema.changeset(%{})

    Enum.reduce(
      records,
      {base_changeset, []},
      fn record, {acc_changeset, acc_records_to_delete} ->
        {changeset, records_to_delete} =
          acc_changeset
          |> merge_plain_attrs(record)
          |> merge_assocs(record, assocs_info, repo)

        {changeset, acc_records_to_delete ++ records_to_delete}
      end
    )
  end

  defp merge_plain_attrs(changeset, record) do
    plain_attrs =
      record
      |> Map.from_struct
      |> Map.delete(:id)
      |> Utils.cleanup_changes

    Changeset.change(changeset, plain_attrs)
  end

  defp get_schema([head | _tail]), do: get_schema(head)
  defp get_schema(%{__struct__: schema}), do: schema

  defp delete(%Ecto.Multi{} = multi, records_to_delete) do
    records_to_delete
    |> Enum.with_index
    |> Enum.reduce(multi, fn({record, i}, acc_multi) ->
      Multi.delete(acc_multi, :"delete_extra_record_#{i}", record)
    end)
  end
end