Find or Create in Ecto

find_or_create.exs

# two different ways to implement a find_or_create for Ecto

find_or_create_user = fn user ->
  case Repo.all(from u in users, where: u.id == ^user.id and u.email == ^user.email) do
    [] ->
      %User{}
      |> User.changeset(user)
      |> Repo.insert!()
    _ ->
      IO.puts "Already inserted"
  end
end

def find_or_create(user) do
  query = from u in users,
          where: u.uid == ^user.uid
  if !Repo.one(query)  do
    Repo.insert(user)
  end
  Repo.one(query)
end

Do these have a race condition?

Yeah, these are not safe implementations.

This is what I would do:

def find_or_create(user_params) do
  {:ok, user} = %User{}
    |> User.changeset(user_params)
    |> Repo.insert(on_conflict: :nothing)

  if is_nil(user.id) do
    Repo.one from u in User, where: u.id == ^user.id
  else
    user
  end
end

With a unique constraint on the user table (probably something like email). This uses the on_conflict option to ensure the record exists before selecting, which should avoid race conditions: https://hexdocs.pm/ecto/Ecto.Repo.html#c:insert/2-upserts

@protestContent this is better but still not the exactly correct behavior. At least on Postgres the serial autoincrement id increases even with on_conflict: :nothing, so with enough find_or_create calls this will needlessly increase autoincrement IDs by quite a bit.

@wyozi good point, I didn't realize that. Still better than a race condition imo. I wonder what the postgres community recommends in cases like this.