Prevent the creation of a garden with a negative number for the area. If entered, the area must be greater than or equal to zero.

app/models/garden.rb

@@ -26,7 +26,7 @@ class Garden < ActiveRecord::Base
     }
 
   validates :area,
-    :numericality => { :only_integer => false },
+    :numericality => { :only_integer => false, :greater_than_or_equal_to => 0 },
     :allow_nil => true
 
   AREA_UNITS_VALUES = {

spec/features/gardens_spec.rb

@@ -33,6 +33,15 @@ feature "Planting a crop", :js => true do
     expect(page).to have_content "New garden"
   end
 
+  scenario "Refuse to create new garden with negative area" do
+    visit new_garden_path
+    fill_in "Name", :with => "Negative Garden"
+    fill_in "Area", :with => -5
+    click_button "Save"
+    expect(page).not_to have_content "Garden was successfully created"
+    expect(page).to have_content "Area must be greater than or equal to 0"
+  end
+
   scenario "Edit garden" do
     visit new_garden_path
     fill_in "Name", :with => "New garden"