Add rake task to move all users from one organisation to another

At the moment, we can only bulk move users from one organisation to
another by uploading a CSV file with their email addresses.

There may be circumstances in which we need to move all the users from
one department to another. For example the motivation to add this task
is that some users have been incorrectly assigned to an older, deleted
instance of an organisation.

Adding a rake task to enable us to do this without needing a CSV file.

lib/tasks/data_hygiene.rake

@@ -10,4 +10,15 @@ namespace :data_hygiene do
   task :close_organisation, %i[content_id] => :environment do |_, args|
     Organisation.find_by(content_id: args[:content_id]).update!(closed: true)
   end
+
+  desc "Move all users from one organisation to another."
+  task :bulk_update_user_organisation, %i[old_content_id new_content_id] => :environment do |_, args|
+    old_organisation = Organisation.find_by(content_id: args[:old_content_id])
+    new_organisation = Organisation.find_by(content_id: args[:new_content_id])
+
+    users = User.where(organisation: old_organisation)
+    users.update_all(organisation_id: new_organisation.id)
+
+    puts "Moved #{users.count} users from #{old_organisation.name} to #{new_organisation.name}"
+  end
 end

test/lib/tasks/data_hygiene_test.rb

@@ -18,4 +18,20 @@ class DataHygieneTaskTest < ActiveSupport::TestCase
       puts "Marked organisation #{organisation.name} as closed"
     end
   end
+
+  context "#bulk_update_user_organisation" do
+    should "update the organisation for matching users" do
+      old_organisation = create(:organisation, slug: "department-of-health-old")
+      new_organisation = create(:organisation, slug: "department-of-health-new")
+      another_organisation = create(:organisation, slug: "department-of-other-stuff")
+
+      user_1 = create(:user, organisation: old_organisation)
+      user_2 = create(:user, organisation: another_organisation)
+
+      Rake::Task["data_hygiene:bulk_update_user_organisation"].invoke(old_organisation.content_id, new_organisation.content_id)
+
+      assert_equal new_organisation, user_1.reload.organisation
+      assert_equal another_organisation, user_2.reload.organisation
+    end
+  end
 end