Spec the failure cases for PreferencesController#update

app/controllers/profiles/preferences_controller.rb

@@ -5,10 +5,15 @@ class Profiles::PreferencesController < Profiles::ApplicationController
   end
 
   def update
+    begin
       if @user.update_attributes(preferences_params)
         flash[:notice] = 'Preferences saved.'
       else
-      # TODO (rspeicher): There's no validation on these values, so can it fail?
+        flash[:alert] = 'Failed to save preferences.'
+      end
+    rescue ArgumentError => e
+      # Raised when `dashboard` is given an invalid value.
+      flash[:alert] = "Failed to save preferences (#{e.message})."
     end
 
     respond_to do |format|

spec/controllers/profiles/preferences_controller_spec.rb

@@ -51,8 +51,24 @@ describe Profiles::PreferencesController do
       end
     end
 
-    context 'on unsuccessful update' do
-      # TODO (rspeicher): Can this happen?
+    context 'on failed update' do
+      it 'sets the flash' do
+        expect(user).to receive(:update_attributes).and_return(false)
+
+        go
+
+        expect(flash[:alert]).to eq('Failed to save preferences.')
+      end
+    end
+
+    context 'on invalid dashboard setting' do
+      it 'sets the flash' do
+        prefs = {dashboard: 'invalid'}
+
+        go params: prefs
+
+        expect(flash[:alert]).to match(/\AFailed to save preferences \(.+\)\.\z/)
+      end
     end
 
     context 'as js' do