Custom validations for changeset Elixir/Phoenix

Question

I have a league model and the league has_many Teams. I only want a user to be able to create one team per league. I'm finding this validation to be tricky. Here is what I'm currently trying.

 def changeset(%Team{} = team, attrs \\ %{}) do
    team
    |> cast(attrs, [:name, :league_id, :user_id])
    |> validate_required([:name, :league_id, :user_id])
    |> one_team_per_user_for_leagues
  end

  defp one_team_per_user_for_leagues(team) do
    if team.changes == %{} do
      team
    else
      league = Repo.get!(League, team.changes[:league_id]) |> Repo.preload(:teams)
      Enum.map(league.teams, fn(team) -> team = team end) |> Enum.any?
    end
  end

But I'm getting this error: no function clause matching in Ecto.Repo.Schema.insert/4

Full stacktrace:

Request: POST /teams
** (exit) an exception was raised:
    ** (FunctionClauseError) no function clause matching in Ecto.Repo.Schema.insert/4
        (ecto) lib/ecto/repo/schema.ex:157: Ecto.Repo.Schema.insert(Statcasters.Repo, Ecto.Adapters.Postgres, false, [])
        (statcasters) lib/statcasters_web/controllers/team_controller.ex:21: StatcastersWeb.TeamController.create/2
        (statcasters) lib/statcasters_web/controllers/team_controller.ex:1: StatcastersWeb.TeamController.action/2
        (statcasters) lib/statcasters_web/controllers/team_controller.ex:1: StatcastersWeb.TeamController.phoenix_controller_pipeline/2
        (statcasters) lib/statcasters_web/endpoint.ex:1: StatcastersWeb.Endpoint.instrument/4
        (phoenix) lib/phoenix/router.ex:278: Phoenix.Router.__call__/1
        (statcasters) lib/statcasters_web/endpoint.ex:1: StatcastersWeb.Endpoint.plug_builder_call/2
        (statcasters) lib/plug/debugger.ex:99: StatcastersWeb.Endpoint."call (overridable 3)"/2
        (statcasters) lib/statcasters_web/endpoint.ex:1: StatcastersWeb.Endpoint.call/2
        (plug) lib/plug/adapters/cowboy/handler.ex:15: Plug.Adapters.Cowboy.Handler.upgrade/4
        (cowboy) /Users/cameronbass/Desktop/Play/statcasters/deps/cowboy/src/cowboy_protocol.erl:442: :cowboy_protocol.execute/4

This also doesn't feel like the "Elixir Way" Can anyone help me untangle this?

Aleksei Matiushkin Aleksei Matiushkin · Accepted Answer · 2017-12-10T05:28:25

Ecto.Changeset.cast/4 returns Ecto.Changeset, as well as all functions piped afterwards, like validate. Your one_team_per_user_for_leagues/1 should also conform this rule:

defp one_team_per_user_for_leagues(%Ecto.Changeset{} = changes) do
  ...
end

Another glitch is how one notifies Ecto about anything wrong: it’s to be done with returning non-empty errors value in the Ecto.Changeset struct:

defp one_team_per_user_for_leagues(%Ecto.Changeset{} = changes) do
  case get_or_create_teams_in_this_league() do # to implement
    {:existing, %Team{}} ->
       new_errors = ...
       %{changes | errors: new_errors ++ changes.errors, valid?: false}
    {:new, %Team{}} ->
       changes
       |> put_assoc(:team, ...)
  end 
end

Custom validations for changeset Elixir/Phoenix

Full stacktrace:

1 Answers