🧹 Marketplace: return only Marketplaces!

- #831
- #709

There's something appealing about switching to Rails' built in `Single
Table Inheritance` for `Furniture`, rather than our flyweight registry;
but I also know that STI can be a bit of a smell...

Anyway, now when you call `Marketplace.all` and get *only*
`Marketplace`s. Maybe someday we'll pull this up in to the `Furniture`
classes responsibilities so we don't have to do it on every single piece
of Furniture...

app/furniture/marketplace/marketplace.rb

@@ -3,6 +3,7 @@
 class Marketplace
   class Marketplace < Furniture
     location(parent: :room)
+    default_scope { where(furniture_kind: "marketplace") }
 
     has_many :products, inverse_of: :marketplace, dependent: :destroy
     has_many :carts, inverse_of: :marketplace, dependent: :destroy

spec/furniture/marketplace/marketplace_spec.rb

@@ -16,4 +16,14 @@
       specify { expect { marketplace.destroy }.not_to change(orders, :count) }
     end
   end
+
+  describe ".all" do
+    subject(:all) { described_class.all }
+
+    let!(:non_marketplace_furniture) { create(:journal) }
+    let!(:marketplace_furniture) { create(:marketplace) }
+
+    it { is_expected.not_to include(non_marketplace_furniture) }
+    it { is_expected.to include(marketplace_furniture) }
+  end
 end