Use a single query in dossiers_id_with_notifications (instead of four)

This is used in /procedures#show and /procedures#index, to display badges on the “suivis” and “traités” tabs of each procedure. Rails cache helps when it’s the exact same query, but it’s not the case for different tabs.

I’m not certain it’ll be a visible performance improvement but it shouldn’t hurt.

app/models/instructeur.rb

@@ -145,30 +145,26 @@ class Instructeur < ApplicationRecord
   def dossiers_id_with_notifications(dossiers)
     dossiers = dossiers.followed_by(self)
 
-    updated_demandes = dossiers
-      .joins(:champs)
+    # Relations passed to #or must be “structurally compatible”, i.e. query the same tables.
+    joined_dossiers = dossiers
+      .left_outer_joins(:champs, :champs_private, :avis, :commentaires)
+
+    updated_demandes = joined_dossiers
       .where('champs.updated_at > follows.demande_seen_at')
 
-    updated_annotations = dossiers
-      .joins(:champs_private)
-      .where('champs.updated_at > follows.annotations_privees_seen_at')
+    # We join `:champs` twice, the second time with `has_many :champs_privates`. ActiveRecord generates the SQL: 'LEFT OUTER JOIN "champs" "champs_privates_dossiers" ON …'. We can then use this `champs_privates_dossiers` alias to disambiguate the table in this WHERE clause.
+    updated_annotations = joined_dossiers
+      .where('champs_privates_dossiers.updated_at > follows.annotations_privees_seen_at')
 
-    updated_avis = dossiers
-      .joins(:avis)
+    updated_avis = joined_dossiers
       .where('avis.updated_at > follows.avis_seen_at')
 
-    updated_messagerie = dossiers
-      .joins(:commentaires)
+    updated_messagerie = joined_dossiers
       .where('commentaires.updated_at > follows.messagerie_seen_at')
       .where.not(commentaires: { email: OLD_CONTACT_EMAIL })
       .where.not(commentaires: { email: CONTACT_EMAIL })
 
-    [
-      updated_demandes,
-      updated_annotations,
-      updated_avis,
-      updated_messagerie
-    ].flat_map { |query| query.distinct.ids }.uniq
+    updated_demandes.or(updated_annotations).or(updated_avis).or(updated_messagerie).ids
   end
 
   def mark_tab_as_seen(dossier, tab)