Creating Autosumming Fields

I’ve created an Opponent model that has several fields associated with it, each of which represents an actual or computed value. You can check out the whole file as it exists right now here, but the schema itself looks like this:

  schema "opponents" do
    field :name,                :string
    field :external_id,         :string
    field :away_losses,         :float
    field :away_wins,           :float
    field :home_losses,         :float
    field :home_wins,           :float
    field :losses,              :float
    field :neutral_losses,      :float
    field :neutral_wins,        :float
    field :opp_opp_winning_pct, :float
    field :opp_winning_pct,     :float
    field :winning_percentage,  :float
    field :wins,                :float

    belongs_to :dataset, Scorcerer.Datasets.Dataset

    timestamps()
  end

Some of the fields are dependent on each other - for example for there to be a winning_percentage there have to be wins and losses. Others can bet set or not and it doesn’t really matter that much - we can compute wins from home_wins and away_wins even if neutral_wins is nil. Right now I’ve got it setup to handle all of that via functions like this:

  def autosum_fields(opponent) do
    if opponent.wins == nil do
      set_wins(opponent)
    end

    if opponent.losses == nil do
      set_losses(opponent)
    end

    if opponent.winning_percentage == nil do
      set_winning_percentage(opponent)
    end
  end

  defp set_winning_percentage(opp) do
    if (opp.wins != nil && opp.losses != nil) do
      Scorcerer.Opponents.update_opponent(opp, %{ winning_percentage: opp.wins / opp.losses })
    end
  end

  defp set_wins(opp) do
    wins = Enum.reduce([:home_wins, :away_wins, :neutral_wins], 0, fn key, acc ->
      key_value = Map.get opp, key
      acc + (key_value || 1)
    end)

    Scorcerer.Opponents.update_opponent(opp, %{ wins: wins })
  end

  defp set_losses(opp) do
    losses = Enum.reduce([:home_losses, :away_losses, :neutral_losses], 0, fn key, acc ->
      key_value = Map.get opp, key
      acc + (key_value || 1)
    end)

    Scorcerer.Opponents.update_opponent(opp, %{ losses: losses })
  end

I’ve actually got two questions here:

1. What is the right way to ensure that my autosum_fields method runs whenever the opponent is updated?
2. Is there a better way to handle the actual updates? I know I’m doing more updates than is required right now because it’s happening on a per-field basis.