Update Postgres table using Ecto/Elixir

Question

I have been learning Elixir lately and am wondering what is the correct syntax for solving my current issue. I am trying to update a model using Elixir and this update includes adding a value to the current value (ie amount = amount + passed_amount) and pushing a new value to an ecto array (ie transactions ++ new_transaction). Here is my latest attempt at doing this:

def add_transaction(conn, %{"coin" => coin_params}) do
        coin = Repo.get_by!(WalletCoin, ticker: coin_params["ticker"])
        coin["transactions"] ++ coin_params["transaction"]
        coin["amount"] = coin["amount"] + coin_params["amount"]
        case Repo.update(coin) do
            {ok, _coin} ->
              coins = Repo.all(WalletCoin)
              render conn, "index.json", coins: coins
        end
end

My intentions of asking this question was to get the answer and hopefully get some insight into Elixir best practices for doing this.

Thanks in advance

Dogbert Dogbert · Accepted Answer · 2018-08-29T18:00:11

Values in Elixir are immutable. You can't just assign a new value to a field in a map like that. You'll first need to create a changeset and then change the two values:

coin =
  Repo.get_by!(WalletCoin, ticker: coin_params["ticker"])

coin = coin
  |> Ecto.Changeset.change()
  |> Ecto.Changeset.put_change(:transactions, coin.transactions ++ coin_params["transaction"])
  |> Ecto.Changeset.put_change(:amount, coin.amount + coin_params["amount"])

Calling Repo.update with this should now work.

Update Postgres table using Ecto/Elixir

1 Answers