Refactor code related to authentication:

* User model
* SessionsController

spec/controllers/sessions_controller_spec.rb

@@ -8,31 +8,39 @@ describe SessionsController do
   end
 
   describe 'POST create' do
-    context 'when the user submit invalid credentials' do
-      it 'renders the new template' do
-        User.stub(:authenticate).and_return(false)
-        post :create,
-          :session => Factory.attributes_for(:user)
-        response.should render_template('new')
-      end
+    let(:user) { Factory.create(:user) }
+
+    def do_create
+      post :create, :session => {
+        :email    => user.email,
+        :password => user.password
+      }
     end
 
-    context 'when the user submit valid credentials' do
-      let(:user) { Factory.create(:user) }
-      before do
-        User.stub(:authenticate).and_return(user)
-      end
-
+    context 'with valid credentials' do
       it 'signs the user in' do
-        post :create, :session => Factory.attributes_for(:user)
+        do_create
         controller.current_user.should == user
       end
 
       it 'redirects to the home page' do
-        post :create, :session => Factory.attributes_for(:user)
+        do_create
         response.should redirect_to(:root)
       end
     end
+
+    [:email, :password].each do |attr|
+      context "with invalid credentials (#{attr})" do
+        before do
+          user.stub(attr => user.send(attr) + '_INVALID')
+        end
+
+        it 'renders the new template' do
+          do_create
+          response.should render_template('new')
+        end
+      end
+    end
   end
 
   describe 'DELETE destroy' do