Add default_card method to user model

app/models/spree/user_decorator.rb

@@ -73,6 +73,10 @@ Spree.user_class.class_eval do
     owned_enterprises(:reload).size < enterprise_limit
   end
 
+  def default_card
+    credit_cards.where(is_default: true).first
+  end
+
   private
 
   def limit_owned_enterprises

spec/models/spree/user_spec.rb

@@ -126,4 +126,32 @@ describe Spree.user_class do
       end
     end
   end
+
+  describe "default_card" do
+    let(:user) { create(:user) }
+
+    context "when the user has no credit cards" do
+      it "returns nil" do
+        expect(user.default_card).to be nil
+      end
+    end
+
+    context "when the user has one credit card" do
+      let!(:card) { create(:credit_card, user: user) }
+
+      it "should be assigned as the default and be returned" do
+        expect(card.reload.is_default).to be true
+        expect(user.default_card.id).to be card.id
+      end
+    end
+
+    context "when the user has more than one card" do
+      let!(:non_default_card) { create(:credit_card, user: user) }
+      let!(:default_card) { create(:credit_card, user: user, is_default: true) }
+
+      it "returns the card which is specified as the default" do
+        expect(user.default_card.id).to be default_card.id
+      end
+    end
+  end
 end