ecto_query_parser

defmodule EctoQueryParser.JoinResolver do
  @moduledoc """
  Resolves dotted identifier paths (e.g., `author.name`, `post.author.company.name`)
  into Ecto dynamic expressions with named bindings and join specifications.
  """

  import Ecto.Query

  @doc """
  Resolves a dotted identifier name against a schema, returning a dynamic
  expression referencing the final field and a list of join specs needed.

  ## Join spec format

      %{binding: atom, assoc: atom, parent: :root | atom}

  ## Examples

      iex> resolve("author.name", MyApp.Post)
      {:ok, dynamic, [%{binding: :author, assoc: :author, parent: :root}]}

      iex> resolve("author.company.company_name", MyApp.Post)
      {:ok, dynamic, [
        %{binding: :author, assoc: :author, parent: :root},
        %{binding: :author__company, assoc: :company, parent: :author}
      ]}
  """
  def resolve(dotted_name, schema) do
    segments = String.split(dotted_name, ".")
    assoc_segments = Enum.slice(segments, 0..-2//1)
    field_name = List.last(segments)

    case walk_associations(assoc_segments, schema, :root, [], []) do
      {:ok, joins, final_binding} ->
        field_atom = String.to_atom(field_name)
        binding_atom = final_binding

        expr = dynamic([{^binding_atom, x}], field(x, ^field_atom))
        {:ok, expr, joins}

      {:error, _} = error ->
        error
    end
  end

  defp walk_associations([], _schema, current_binding, _path, joins) do
    {:ok, Enum.reverse(joins), current_binding}
  end

  defp walk_associations([segment | rest], schema, parent, path, joins) do
    assoc_atom = String.to_atom(segment)

    case schema.__schema__(:association, assoc_atom) do
      nil ->
        {:error, "unknown association: #{segment} on #{inspect(schema)}"}

      assoc ->
        new_path = path ++ [segment]
        binding = new_path |> Enum.join("__") |> String.to_atom()

        join_spec = %{binding: binding, assoc: assoc_atom, parent: parent}
        next_schema = assoc.queryable

        walk_associations(rest, next_schema, binding, new_path, [join_spec | joins])
    end
  end
end