A simple example of how the magic of Active Record can sometimes be misused

active_record_and_count_v_length.txt

# Length acts as an array method and gives the correct record count. 
1.9.3-p362 :028 > u.clicks.select(:user_id).uniq.length
  Click Load (0.4ms)  SELECT DISTINCT user_id FROM "clicks" WHERE "clicks"."url_id" = 4
 => 2 

# Here .count is being used by Active Record and inserted into the generated SQL and is not an array method.  It gives a wrong answer.
1.9.3-p362 :029 > u.clicks.select(:user_id).uniq.count
   (0.4ms)  SELECT DISTINCT COUNT("clicks"."user_id") FROM "clicks" WHERE "clicks"."url_id" = 4
 => 3 

# Correct use of .count via Active Record
1.9.3-p362 :027 > u.clicks.select("DISTINCT(clicks.user_id)").count
   (0.4ms)  SELECT COUNT(DISTINCT(clicks.user_id)) FROM "clicks" WHERE "clicks"."url_id" = 4
 => 2