Just rely on the constraints to avoid duplicate Follows

Don’t check manually in advance: just try to create the new Follow and silently fail.

Since we have both Rails validation and DB constraints in place, we have two types of errors to check. I’m not actually sure this change improves the legibility of the code.

app/models/gestionnaire.rb

@@ -27,21 +27,15 @@ class Gestionnaire < ApplicationRecord
   end
 
   def follow(dossier)
-    if follow?(dossier)
-      return
-    end
-
     begin
       followed_dossiers << dossier
+      # If the user tries to follow a dossier she already follows,
+      # we just fail silently: it means the goal is already reached.
     rescue ActiveRecord::RecordNotUnique
-      # Altough we checked before the insertion that the gestionnaire wasn't
-      # already following this dossier, this was done at the Rails level:
-      # at the database level, the dossier was already followed, and a
-      # "invalid constraint" exception is raised.
-      #
-      # We can ignore this safely, as it means the goal is already reached:
-      # the gestionnaire follows the dossier.
-      return
+      # Database uniqueness constraint
+    rescue ActiveRecord::RecordInvalid => e
+      # ActiveRecord validation
+      raise unless e.record.errors.details.dig(:gestionnaire_id, 0, :error) == :taken
     end
   end