As Zabba said, you cannot access current_userin your model. You can pass an argument to your scope:
scope :user_answers, lambda {|user_id| where(:user_id => user_id)}
Then you can call a scope like this (assuming you are calling it from a controller or view):
Answer.user_answers(current_user.id)
Update
, , - :
scope :qs_w_user_ans, lambda {|user_id| joins(:questions) & Answer.user_answers(user_id)}
current_user.id, qs_w_user_ans. user_id user_answers, . ( ).