uninitialized constant FacebookController::OAuth2

[reiz]

I get this message:

"uninitialized constant FacebookController::OAuth2"

My Controller looks like this:

def start
redirect client.web_server.authorize_url(
:redirect_uri => redirect_uri,
:scope => 'email,offline_access'
)
end

def callback
access_token = client.web_server.get_access_token(params[:code], :redirect_uri => redirect_uri)
user = JSON.parse(access_token.get('/me'))
user.inspect
end

def client
OAuth2::Client.new('id',
'secret',
:site => 'https://graph.facebook.com')
end

def redirect_uri
uri = URI.parse(request.url)
uri.path = '/auth/facebook/callback'
uri.query = nil
uri.to_s
end