Destroy in reverse order to rspect foreign keys (#478)

Otherwise a foreign key error can occur if foreign keys have been added,
for example:

`Mysql2::Error: Cannot delete or update a parent row: a foreign key constraint fails`

lib/awesome_nested_set/model/prunable.rb

@@ -35,9 +35,14 @@ def destroy_descendants
             end
           end
 
+          # Use reverse to delete from deepest child to parent in order to respect any possible foreign keys
+          def decendants_to_destroy_in_order
+            descendants.reverse
+          end
+
           def destroy_or_delete_descendants
             if acts_as_nested_set_options[:dependent] == :destroy
-              descendants.each do |model|
+              decendants_to_destroy_in_order.each do |model|
                 model.skip_before_destroy = true
                 model.destroy
               end

spec/models/users_spec.rb

@@ -652,6 +652,19 @@
     expect(User.valid?).to be_truthy
   end
 
+  it "destroys in the right order to respect foreign keys" do
+    User.acts_as_nested_set_options[:dependent] = :destroy
+
+    expect(users(:top_level).decendants_to_destroy_in_order).to eq [
+      users(:child_3),
+      users(:child_2_1),
+      users(:child_2),
+      users(:child_1)
+    ]
+    expect(users(:top_level)).to receive(:decendants_to_destroy_in_order).once.and_call_original
+    expect { users(:top_level).destroy! }.to change(User, :count).by(-5)
+  end
+
   it "assigning_parent_uuid_on_create" do
     user = User.create!(:name => "Child", :parent_uuid => users(:child_2).uuid)
     expect(users(:child_2)).to eq(user.parent)