Associating Two Records in a Many-to-Many Relationship

update.ex

defmodule Identity do
  # ...
  
  defp link_user_and_company(user = %User{}, company = %Company{}) do
    user = Repo.preload(user, :companies)
    companies = user.companies ++ [company]
                |> Enum.map(&Ecto.Changeset.change/1)
                
    user
    |> Ecto.Changeset.change
    |> Ecto.Changeset.put_assoc(:companies, companies)
    |> Repo.update
  end
  
  # ...
end