Fix managing widget cards for homepage

The condition `params[:page_id] != 0` didn't work properly when editing
the homepage because in that case the parameter was `nil`, and the line
`SiteCustomization::Page.find(@card.site_customization_page_id)` raised
an exception because it couldn't find a page with a `nil` ID.

Fixing the issue while maintaining the check against `0` lead to complex
code, and so allowing `nil` in the database and assuming cards with no
`site_customization_page_id` belonged in the homepage seemed to be the
easiest solution.

app/controllers/admin/widget/cards_controller.rb

@@ -4,10 +4,8 @@ class Admin::Widget::CardsController < Admin::BaseController
   def new
     if header_card?
       @card = ::Widget::Card.new(header: header_card?)
-    elsif params[:page_id] != 0
-      @card = ::Widget::Card.new(site_customization_page_id: params[:page_id])
     else
-      @card = ::Widget::Card.new
+      @card = ::Widget::Card.new(site_customization_page_id: params[:page_id])
     end
   end
 
@@ -59,7 +57,7 @@ class Admin::Widget::CardsController < Admin::BaseController
   def redirect_to_customization_page_cards_or_homepage
     notice = t("admin.site_customization.pages.cards.#{params[:action]}.notice")
 
-    if params[:page_id] != 0
+    if @card.site_customization_page_id
       redirect_to admin_site_customization_page_cards_path(page), notice: notice
     else
       redirect_to admin_homepage_url, notice: notice

app/models/widget/card.rb

@@ -16,7 +16,7 @@ class Widget::Card < ActiveRecord::Base
   end
 
   def self.body
-    where(header: false, site_customization_page_id: 0).order(:created_at)
+    where(header: false, site_customization_page_id: nil).order(:created_at)
   end
 
   #add widget cards to custom pages

db/migrate/20181218164126_add_site_customization_page_to_widget_cards.rb

@@ -1,5 +1,5 @@
 class AddSiteCustomizationPageToWidgetCards < ActiveRecord::Migration
   def change
-    add_reference :widget_cards, :site_customization_page, index: true, default: 0
+    add_reference :widget_cards, :site_customization_page, index: true
   end
 end

db/schema.rb

@@ -1503,7 +1503,7 @@ ActiveRecord::Schema.define(version: 20181218164126) do
     t.boolean  "header",                     default: false
     t.datetime "created_at",                                 null: false
     t.datetime "updated_at",                                 null: false
-    t.integer  "site_customization_page_id", default: 0
+    t.integer  "site_customization_page_id"
   end
 
   add_index "widget_cards", ["site_customization_page_id"], name: "index_widget_cards_on_site_customization_page_id", using: :btree

spec/models/widget/card_spec.rb

@@ -26,11 +26,14 @@ describe Widget::Card do
 
     it "returns cards for the homepage body" do
       header = create(:widget_card, header: true)
-      card1 = create(:widget_card, header: false, title: "Card 1", site_customization_page_id: 0)
-      card2 = create(:widget_card, header: false, title: "Card 2", site_customization_page_id: 0)
-      card3 = create(:widget_card, header: false, title: "Card 3", site_customization_page_id: 0)
+      card1 = create(:widget_card, header: false)
+      card2 = create(:widget_card, header: false)
+      page_card = create(:widget_card, header: false, page: create(:site_customization_page))
 
-      expect(Widget::Card.body).to eq([card1, card2, card3])
+      expect(Widget::Card.body).to include(card1)
+      expect(Widget::Card.body).to include(card2)
+      expect(Widget::Card.body).not_to include(header)
+      expect(Widget::Card.body).not_to include(page_card)
     end
   end