You need support for testing sessions when, for example, testing authentication. The only way I've managed to get sessions to work with Sinatra and Rack::Test is by going through the whole stack, in other words calling the authentication controller as shown here:

@user = Factory(:user) # create a dummy user
User.expects(:authenticate).with(any_parameters).returns(@user) # make authenticate return the dummy user
post "/sign-in", {:email => @user.email, :password => @user.password} # login to the application and set a session variable.

After this the session populated and we're logged in when accessing the application again:

post "/articles", {:title => 'Sessions suck', :body => '...'}

On a side note, there are two ways of specifying sessions that used to work, but which no longer work with Sinatra 1.0:

get '/', :env => { :session => {:abc => 'adf'} }
get '/', {}, :session => {:abc => 'adf'}
get '/', {}, "rack.session" => {:abc => 'adf'}

The session is always empty.

There are many discussions about sessions and Rack::Test, but not one of them has a solution that works for me: * rack.session variable is missing env under test environment * Sessions with rspec and Rack.Test

Mocking with expectations:

@domain = Domain.new
Domain.expects(:new).returns(@domain)
@domain.errors.add(:name, 'WTF')

Stubbing:

Domain.stubs(:new).returns(@domain)