fix: disallow attendees to have multiple credentials

lib/safira/accounts.ex

@@ -1030,24 +1030,6 @@ defmodule Safira.Accounts do
     Course.changeset(course, attrs)
   end
 
-  @doc """
-  Gets a single credential associated to the given attendee.
-
-  ## Examples
-
-      iex> get_credential_of_attendee(%Attendee{})
-      %Credential{}
-
-      iex> get_credential_of_attendee!(%Attendee{})
-      nil
-
-  """
-  def get_credential_of_attendee(%Attendee{} = attendee) do
-    Credential
-    |> where([c], c.attendee_id == ^attendee.id)
-    |> Repo.one()
-  end
-
   def generate_credentials(count) do
     for _ <- 1..count do
       {:ok, credential} = create_credential(%{})

lib/safira/accounts/credential.ex

@@ -14,6 +14,7 @@ defmodule Safira.Accounts.Credential do
   def changeset(credential, attrs) do
     credential
     |> cast(attrs, [:attendee_id])
+    |> unique_constraint(:attendee_id)
     |> cast_assoc(:attendee)
   end
 end

lib/safira_web/plugs/user_roles.ex

@@ -53,7 +53,7 @@ defmodule SafiraWeb.UserRoles do
 
   defp has_credential?(conn) do
     is_nil(conn.assigns.current_user.attendee) or
-      not is_nil(Accounts.get_credential_of_attendee(conn.assigns.current_user.attendee))
+      Accounts.attendee_has_credential?(conn.assigns.current_user.attendee.id)
   end
 
   @doc """

priv/repo/migrations/20240919141626_create_credentials.exs

@@ -9,6 +9,6 @@ defmodule Safira.Repo.Migrations.CreateCredentials do
       timestamps(type: :utc_datetime)
     end
 
-    create index(:credentials, [:attendee_id])
+    create unique_index(:credentials, [:attendee_id])
   end
 end