impressions_by_day.rb

article.impressions.create
article.impressions.create(:created_at => "2000-01-01")
article.impressions.create(:created_at => "2000-01-01")
article.impressions.create(:created_at => "2000-01-02")

impressions = article.impressions.select("count(*) as count, DATE_FORMAT(created_at,'%Y-%m-%d') as created_day").group("created_day")

impressions.each do |impression|
  p "#{impression.created_day}: #{impression.count}"
end

This should output:

"2000-01-01: 2"
"2000-01-02: 1"
"2012-08-17: 1"