Fix the way in which the conditions are queried and key is set

Thanks Ivan!

db/migrate/20240909151818_migrate_legacy_default_theme_to_light_theme.rb

@@ -29,29 +29,29 @@
 #++
 
 class MigrateLegacyDefaultThemeToLightTheme < ActiveRecord::Migration[7.1]
-  class MigrationUserPreference < ActiveRecord::Base
-    self.table_name = "user_preferences"
-  end
-
   # Migrate legacy `default` theme to `light` theme
   #
   # Moving from:
   # {
   #   "settings": {
-  #     "theme": "default"
+  #     "theme": "default",
+  #     "foo": "bar"
   #   }
   # }
   #
   # To:
   # {
   #   "settings": {
-  #     "theme": "light" # current default theme
+  #     "theme": "light", # current default theme
+  #     "foo": "bar"
   #   }
   # }
   def up
-    MigrationUserPreference
-      .where(settings: { theme: "default" })
-      .update_all(settings: { theme: "light" })
+    execute <<-SQL.squish
+      UPDATE user_preferences
+      SET settings = jsonb_set(settings, '{theme}', '"light"')
+      WHERE settings->>'theme' = 'default';
+    SQL
   end
 
   # no-op

spec/migrations/migrate_legacy_default_theme_to_light_theme_spec.rb

@@ -35,10 +35,12 @@
   # Silencing migration logs, since we are not interested in that during testing
   subject(:run_migration) { ActiveRecord::Migration.suppress_messages { described_class.new.up } }
 
-  shared_let(:user_with_default_theme) { create(:user, preferences: { settings: { theme: "default" } }) }
-  shared_let(:user_with_light_theme) { create(:user, preferences: { settings: { theme: "light" } }) }
-  shared_let(:user_with_light_high_contrast_theme) { create(:user, preferences: { settings: { theme: "light_high_contrast" } }) }
-  shared_let(:user_with_dark_theme) { create(:user, preferences: { settings: { theme: "dark" } }) }
+  shared_let(:user_with_default_theme) { create(:user, preferences: { settings: { theme: "default", foo: "bar" } }) }
+  shared_let(:user_with_light_theme) { create(:user, preferences: { settings: { theme: "light", foo: "bar" } }) }
+  shared_let(:user_with_light_high_contrast_theme) do
+    create(:user, preferences: { settings: { theme: "light_high_contrast", foo: "bar" } })
+  end
+  shared_let(:user_with_dark_theme) { create(:user, preferences: { settings: { theme: "dark", foo: "bar" } }) }
 
   it "sets the theme to light for users with the 'default' theme" do
     expect { run_migration }