create_build_service.ex

defmodule Opencov.CreateBuildService do
  def make_changeset(project, params \\ :empty) do
    Ecto.Model.build(project, :builds)
    |> Opencov.Build.changeset(params)
    |> add_previous_values(project)
  end
  
    defp add_previous_values(changeset, project) do
    if previous_build = search_previous_build(changeset, project) do
      change(changeset, %{previous_build_id: previous_build.id, previous_coverage: previous_build.coverage})
    else
      changeset
    end
  end

  defp search_previous_build(changeset, project) do
    Opencov.Build.previous(project.id,
                   get_change(changeset, :build_number),
                   get_change(changeset, :branch))
    |> Opencov.Repo.one
  end
end

factory.ex

defmodule Opencov.Factory do
  use ExMachina.Ecto, repo: Opencov.Repo
  
  def factory(:build) do
    attrs = %{
      build_number: sequence(:build_number, &(&1)),
    }
    changeset = Opencov.CreateBuildService.make_changeset(create(:project), attrs)
    Map.merge(changeset.model, changeset.changes)
  end
end