Hi there. I am trying to implement the logic that does get_by(..) and if not exists, it should insert and return.

The first implementation is like this. (We are using Postgres.)

def get_or_insert(account_id, type) do
  case Repo.get_by(Storage, account_id: account_id, type: type) do
    nil -> Repo.insert(Storage, %{account_id: account_id, type: type})
    storage -> {:ok, storage}
  end
end

The storage has the composite unique index for account_id and type. So it works fine until there are 2 same requests. I expected it to return same storage for both req, but one of them will get error for type already exists because of race condition that the Repo.get_by from 2 req were evaluated at the same time before any insertion.

As I am familiar with Rails, I would solve this in Rails by doing account.lock do ... end to prevent reading the record at the same time. But not sure how to implement this in Ecto. Any idea to implement this idea while avoiding race condition?

So far, this one is the most similar for what I am looking for, but still answer only the case of get(id)