How to sum fields after group_by - ruby-on-rails

Question

I need to group some records by date, then sum one field for each of those days.
This is the code I'm trying to refactor:
<% #startdate.upto(#enddate) do |date| %>
<% visitors = #links.where("created_at >= ? AND created_at < ?", date, date+1).order("created_at ASC").select("id, visit_count") %>
<%= datevisitors.sum("visit_count") || 0 %>
<% end %>
I've started with:
<% visitors = #links.select("visit_count, created_at").group_by{|l| l.created_at.day} %>
<% #startdate.upto(#enddate) do |date| %>
<%= visitors[date] ...?
<% end %>

Answer 1

When you group_by something, you receive an array of the day and the element. So if you group by day, you'll have the Day and the elements that are part of the array:
<% #links.group_by{|l| l.created_at.day}.each do |day, links| %>
<p><%= day %>: <%= links.sum(&:visit_count) %></p>
<% end %>
EDIT --
OK, in that case your code looks like it was very close:
<% visitors = #links.select("visit_count, created_at").group_by{|l| l.created_at.day} %>
<% #startdate.upto(#enddate) do |date| %>
<% if visitors[date] %>
<%= visitors[date].sum(&:visit_count) %>
<% else %>
0
<% end %>
<% end %>
It's probably better to move this to a helper (or presenter)
def daily_visits(links, from, to)
links_by_day = links.select("visit_count, created_at").group_by{|l| l.created_at.day}
range = from.upto(to).to_a
range.inject({}) do |date, hash|
hash[date] = links_by_day.fetch(date, 0)
end
end
Then, in the view:
<% daily_visits(#links, #startdate, #enddate).each do |day, count| %>
<%= day %>: <%= count %>
<% end %>

Answer 2

This should be on controller
#dates_with_visitors = #links.select("visit_count, created_at").group_by(&:created_at)
This should be on the view
<% #dates_with_visitors.each do |date_with_visitors| %>
<%= "#{date_with_visitor[0]} - Visits: #{date_with_visitors[1].sum(&:visit_count)}" %>
<% end %>

Answer 3

Hi can you give me the Model that will be used?
Here's my example on a Activerecord with a aggregate sum function:
<% #sum_visit_count = Visitor.find(:all, :select => "SUM(visit_count)", :group => "id, date_visit") %>
and
<%= #sum_visit_count.sum %>