speedup unique impressions by using "Impression#exists?" instead of "#Impression#size == 0"
speedup unique impressions by using "Impression#exists?" instead of "#Impression#size == 0"

charlotte-ruby/impressionist,charlotte-ruby/impressionist,samchen2009/impressionist,charlotte-ruby/impressionist,guodongme/impressionist,firmanm/impressionist,samchen2009/impressionist,samchen2009/impressionist,firmanm/impressionist,guodongme/impressionist,guodongme/impressionist,firmanm/impressionist