db: add a data migration to remove invalid roles

This prepares a future migration where we can add database constraints
to those roles.

lib/tasks/deployment/20220316105857_delete_roles_without_users.rake

@@ -0,0 +1,78 @@
+namespace :after_party do
+  desc 'Deployment task: delete_roles_without_users'
+  task delete_roles_without_users: :environment do
+    rake_puts "Running deploy task 'delete_roles_without_users'"
+
+    # For now, deleting a user with any role (administrateur, instructeur or expert) is forbidden.
+    #
+    # However, in the past, some users with roles may have been deleted – without the corresponding roles being
+    # also removed. This creates invalid roles, referenced by no users (but still referenced by other records).
+    #
+    # To prevent this situation, future database migrations will add more constraints on roles.
+    # But before adding constraints, we need to clean up any dangling role that may exist.
+
+    # Helper method
+    def raise_no_admin_error(service)
+      message = <<~EOE
+        Error while re-assigning Service \##{service.id} to another Administrateur.
+
+        We tried to re-assign the Service \##{service.id} to another Administrateur,
+        but none of the Procedures (#{service.procedures.pluck(:id)}) attached to this Service had other Administrateurs
+        to assign the service to.
+
+        Please fix the Service \##{service.id} manually (by assigning it to any valid administrator, for instance the
+        first one in your database), then run this task again.
+      EOE
+      raise RuntimeError, message
+    end
+
+    #
+    # Delete dangling administrateurs (administrateurs without an existing user)
+    #
+
+    rake_puts "\nFinding dangling administrateurs…"
+    dangling_administrateurs = Administrateur.where.missing(:user)
+    rake_puts "#{dangling_administrateurs.count} dangling administrateurs found."
+
+    if dangling_administrateurs.any?
+      # An administrateur can't be removed if it has services.
+      # For those services:
+      #   - delete services not referenced by any procedure
+      #   - re-assign the other services to the first admin of a procedure using this service
+      rake_puts "  Removing or re-assigning their services…"
+      dangling_services = dangling_administrateurs.map(&:services).flatten
+      services_with_procedures, services_without_procedures = dangling_services.partition { |s| s.procedures.any? }
+      services_with_procedures.each do |service|
+        other_admins = service.procedures.flat_map(&:administrateurs).excluding(service.administrateur)
+        raise_no_admin_error(service) if other_admins.empty?
+        service.update!(administrateur: other_admins.first)
+      end
+      services_without_procedures.each(&:destroy)
+      rake_puts "  #{services_with_procedures.length} services re-assigned, #{services_without_procedures.length} services destroyed."
+
+      # Now we can destroy dangling administrateurs
+      rake_puts "  Destroying #{dangling_administrateurs.count} dangling administrateurs…"
+      deleted_records = dangling_administrateurs.destroy_all
+      rake_puts "  #{deleted_records.length} dangling administrateurs destroyed."
+    end
+
+    #
+    # Delete dangling instructeurs (instructeurs without an existing user)
+    #
+    rake_puts "\nFinding dangling instructeurs…"
+    deleted_records = Instructeur.where.missing(:user).destroy_all
+    rake_puts "#{deleted_records.length} dangling instructeurs destroyed."
+
+    #
+    # Delete dangling experts (experts without an existing user)
+    #
+    rake_puts "\nFinding dangling experts…"
+    deleted_records = Expert.where.missing(:user).destroy_all
+    rake_puts "#{deleted_records.length} dangling experts destroyed."
+
+    # Update task as completed.  If you remove the line below, the task will
+    # run with every deploy (or every time you call after_party:run).
+    AfterParty::TaskRecord
+      .create version: AfterParty::TaskRecorder.new(__FILE__).timestamp
+  end
+end