DEV: Introduce theme-owned color palettes (#30915)

Related to https://github.com/discourse/discourse/pull/30893

As part of the theme overhauling project, we're making each theme fully
own/control its color palette which can be edited directly on the theme
page. To make this possible, we need to introduce a special type of
color palettes that are marked as "owned by a theme" in the database
which aren't displayed in the admin color palettes page and can't be
edited from it. This commit is the first step of this change; it adds a new
join table to associate a color palette with a theme. For now, we're
keeping the relationship one-to-one (hence the `UNIQUE` indexes), but we
may later change it to one-to-many.

Internal topic: t/141648.

app/models/color_scheme.rb

@@ -312,6 +312,13 @@ class ColorScheme < ActiveRecord::Base
   after_destroy :dump_caches
   belongs_to :theme
 
+  has_one :theme_color_scheme
+  has_one :owning_theme, class_name: "Theme", through: :theme_color_scheme, source: :theme
+
+  default_scope do
+    where("color_schemes.id NOT IN (SELECT color_scheme_id FROM theme_color_schemes)")
+  end
+
   validates_associated :color_scheme_colors
 
   BASE_COLORS_FILE = "#{Rails.root}/app/assets/stylesheets/common/foundation/colors.scss"

app/models/theme.rb

@@ -46,6 +46,12 @@ class Theme < ActiveRecord::Base
           -> { where(target_id: Theme.targets[:settings], name: "yaml") },
           class_name: "ThemeField"
   has_one :javascript_cache, dependent: :destroy
+  has_one :theme_color_scheme, dependent: :destroy
+  has_one :owned_color_scheme,
+          class_name: "ColorScheme",
+          through: :theme_color_scheme,
+          source: :color_scheme
+
   has_many :locale_fields,
            -> { filter_locale_fields(I18n.fallbacks[I18n.locale]) },
            class_name: "ThemeField"

app/models/theme_color_scheme.rb

@@ -0,0 +1,22 @@
+# frozen_string_literal: true
+
+class ThemeColorScheme < ActiveRecord::Base
+  belongs_to :theme
+  belongs_to :color_scheme, dependent: :destroy
+end
+
+# == Schema Information
+#
+# Table name: theme_color_schemes
+#
+#  id              :bigint           not null, primary key
+#  theme_id        :integer          not null
+#  color_scheme_id :integer          not null
+#  created_at      :datetime         not null
+#  updated_at      :datetime         not null
+#
+# Indexes
+#
+#  index_theme_color_schemes_on_color_scheme_id  (color_scheme_id) UNIQUE
+#  index_theme_color_schemes_on_theme_id         (theme_id) UNIQUE
+#

db/migrate/20250121180125_create_theme_color_scheme.rb

@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class CreateThemeColorScheme < ActiveRecord::Migration[7.2]
+  def change
+    create_table :theme_color_schemes do |t|
+      t.integer :theme_id, null: false
+      t.integer :color_scheme_id, null: false
+      t.timestamps null: false
+    end
+
+    add_index :theme_color_schemes, :theme_id, unique: true
+    add_index :theme_color_schemes, :color_scheme_id, unique: true
+  end
+end

spec/models/theme_spec.rb

@@ -1585,4 +1585,15 @@ HTML
       expect(count).to eq 3
     end
   end
+
+  describe "#owned_color_scheme" do
+    it "is destroyed when the theme is destroyed" do
+      scheme = Fabricate(:color_scheme, owning_theme: theme)
+
+      theme.destroy!
+
+      expect(ThemeColorScheme.exists?(color_scheme_id: scheme.id)).to eq(false)
+      expect(ColorScheme.unscoped.exists?(id: scheme.id)).to eq(false)
+    end
+  end
 end

spec/requests/admin/color_schemes_controller_spec.rb

@@ -45,6 +45,18 @@ RSpec.describe Admin::ColorSchemesController do
         expect(scheme_colors[0]["name"]).to eq("primary")
         expect(scheme_colors[0]["hex"]).to eq(scheme.resolved_colors["primary"])
       end
+
+      it "doesn't list theme-owned color schemes" do
+        owned_scheme = Fabricate(:color_scheme, owning_theme: Fabricate(:theme))
+        scheme = Fabricate(:color_scheme)
+
+        get "/admin/color_schemes.json"
+        expect(response.status).to eq(200)
+
+        ids = response.parsed_body.map { |obj| obj["id"] }
+        expect(ids).to include(scheme.id)
+        expect(ids).not_to include(owned_scheme.id)
+      end
     end
 
     shared_examples "color schemes inaccessible" do
@@ -148,6 +160,15 @@ RSpec.describe Admin::ColorSchemesController do
         expect(response.status).to eq(422)
         expect(response.parsed_body["errors"]).to be_present
       end
+
+      it "doesn't allow editing theme-owned schemes" do
+        color_scheme = Fabricate(:color_scheme, owning_theme: Fabricate(:theme))
+
+        put "/admin/color_schemes/#{color_scheme.id}.json", params: valid_params
+        expect(response.status).to eq(404)
+        color_scheme.reload
+        expect(color_scheme.name).not_to eq(valid_params[:color_scheme][:name])
+      end
     end
 
     shared_examples "color scheme update not allowed" do
@@ -184,6 +205,14 @@ RSpec.describe Admin::ColorSchemesController do
         }.by(-1)
         expect(response.status).to eq(200)
       end
+
+      it "doesn't allow deleting theme-owned schemes" do
+        color_scheme = Fabricate(:color_scheme, owning_theme: Fabricate(:theme))
+
+        delete "/admin/color_schemes/#{color_scheme.id}.json"
+        expect(response.status).to eq(404)
+        expect(color_scheme.reload).to be_persisted
+      end
     end
 
     shared_examples "color scheme deletion not allowed" do