Best elixir idiom for "var = this || that"?

[Jauny]

Hi!

I’m newly learning Elixir and just bumped again a problem that I’m not sure how to write, although I feel like there is probably a neat pattern matching solution, or maybe some better FP syntax.

How would you write the following code?

user = Repo.get_by(User, name: "jon")
unless user do
  user = User |> Ecto.Query.first |> Repo.one
end
user

I feel like there is a better Elixir/FP idiom to write this?

Cheers!

[NobbZ]

What is this code expected to do?

And the code you used on the title would work in elixir as is.

[benwilson512]

When in doubt, try!

user = Repo.get_by(User, name: "jon") || User |> Ecto.Query.first |> Repo.one

[Jauny]

Yes sorry if I was not clear. The code works right now, but coming from the ruby world, I was wondering if there was a syntax that would suit Elixir better (aka something for FP).

Seems like not Thanks for the quick replies!

[kokolegorille]

You might search for first_or_create like in this post

Ecto helpers to find_or_create(_by) and upsert(_by)

Hmm, that’s a good point… This will more likely than not cause potential race conditions for the find_or_create(_by) since it’d pass a struct to an insert which could then lead to duplicates. Any ideas on how to handle this? Regarding the upsert(_by), that took inspiration from the example given for Ecto.Repo#insert_or_update/2 by passing a changeset to the insert_or_update. Looking at it again, I’m curious whether the insert_or_update knows how to gracefully handle race conditions …

But beware of race condition.

[Eiji]

@Jauny

Yes sorry if I was not clear. The code works right now, but coming from the ruby world, I was wondering if there was a syntax that would suit Elixir better (aka something for FP).

How about:

defmodule Example do
  def get_user(opts), do: User |> Repo.get_by(opts) |> do_get_user()

  defp do_get_user(nil), do: User |> Ecto.Query.first |> Repo.one
  defp do_get_user(user), do: user
end

[outlog]

def get_user  
  case Repo.get_by(User, name: "jon") do
    nil -> User |> Ecto.Query.first |> Repo.one
    repo_user -> repo_user
  end
end

but like the pattern matching one much better…

also be aware of errors if get_by returns more than one record… so maybe refactor this…

[peerreynders]

@Jauny

aka something for FP

@Jauny

How would you write the following code?

user = Repo.get_by(User, name: "jon")
unless user do
  user = User |> Ecto.Query.first |> Repo.one
end
user

Here unless is being used like a statement - which can result in warnings

FP uses expressions, e.g.:

user = 
  case Repo.get_by(User, name: "jon") do
    nil ->
      User |> Ecto.Query.first |> Repo.one
    result ->
      result
  end

which was already shortened to:

user = Repo.get_by(User, name: "jon") || User |> Ecto.Query.first |> Repo.one

which still is an expression.

[Jauny]

outlog and peerreynders thanks that’s exactly what I was looking for!

[sasajuric]

with is also an option, thought I’d personally likely use || here

user = with nil <- Repo.get_by(User, name: "jon"), do: User |> Ecto.Query.first() |> Repo.one()