Getting count of elements by `created_at` by day in a given month

Question

I wanted to make a simple chart of users that have been created in the past month in my app. Like basically for each day in the past month I want to show the count of users

Answer 1

As @floyd commented on earlier, the code to do the SELECT belongs in the model:

class User < ActiveRecord::Base
  def self.count_new_users_per_day(cutoff_at)
    result = count(:all, :conditions => ["created_at >= ?", cutoff_at],
                         :group => "DATE(created_at)")
    # See http://ruby-doc.org/core-1.8.7/classes/Hash.html#M000163
    result.default = 0
    result
  end
end

The controller does logic and calls into the model layer:

class UsersController < ActionController::Base
  def index
    @cutoff_at = 30.days.ago.at_midnight
    @new_users_by_date = User.count_new_users_per_day(@cutoff_at)
    @dates = ((@cutoff_at.to_date) .. (@cutoff_at.to_date >> 1))
  end
end

And the view is only responsible for displaying the data the controller's setup for it:

# Chose to move the code to a partial
<%= render :partial => "user_count", :collection => @dates, :as => :date %>

# _user_count.html.erb
<%=h date.to_s(:db) %>
<%= number_with_delimiter(@new_users_by_date[date.to_s(:db)]) %>

Basically, since SQL won't return missing dates, you have to iterate over the full set of dates yourself, and ask the Hash/ResultSet if it has the correct value. In the model implementation above, I set the Hash's default value after the fact, giving me a clean way to get a zero when a value is missing.