FIX: properly clean up related color schemes on delete

app/models/theme.rb

@@ -44,6 +44,18 @@ class Theme < ActiveRecord::Base
     if SiteSetting.default_theme_key == self.key
       Theme.clear_default!
     end
+
+    if self.id
+
+      ColorScheme
+        .where(theme_id: self.id)
+        .where("id NOT IN (SELECT color_scheme_id FROM themes where color_scheme_id IS NOT NULL)")
+               .destroy_all
+
+      ColorScheme
+        .where(theme_id: self.id)
+        .update_all(theme_id: nil)
+    end
   end
 
   after_commit ->(theme) do

app/serializers/color_scheme_serializer.rb

@@ -5,4 +5,8 @@ class ColorSchemeSerializer < ApplicationSerializer
   def theme_name
     object.theme&.name
   end
+
+  def theme_id
+    object.theme&.id
+  end
 end

spec/models/theme_spec.rb

@@ -23,6 +23,21 @@ describe Theme do
     expect(s.key).not_to eq(nil)
   end
 
+  it 'can properly clean up color schemes' do
+    theme = Theme.create!(name: 'bob', user_id: -1)
+    scheme = ColorScheme.create!(theme_id: theme.id, name: 'test')
+    scheme2 = ColorScheme.create!(theme_id: theme.id, name: 'test2')
+
+    Theme.create!(name: 'bob', user_id: -1, color_scheme_id: scheme2.id)
+
+    theme.destroy!
+    scheme2.reload
+
+    expect(scheme2).not_to eq(nil)
+    expect(scheme2.theme_id).to eq(nil)
+    expect(ColorScheme.find_by(id: scheme.id)).to eq(nil)
+  end
+
   it 'can support child themes' do
     child = Theme.new(name: '2', user_id: user.id)