Filling in blanks for DATE_TRUNC grouping with rails - ruby-on-rails

Question

The following query to the postgres database
#interventos_2 = Intervento.where(['previsto >= ?', start_date]).group("DATE_TRUNC('week', previsto)").count
generates a hash from which the view can extract the data as follows
<% #interventos_2.each do |w| %>
<%= w[0].strftime('%Y-%W') %> <%= w[1] %><br />
<% end %>
However if there is a blank (count = 0) in the range of weeks we are concerned with, the sequence of years and commercial weeks will look weird and or misleading.
What is an efficient way to declare the range and then fill in the blank weeks with zero?
Update the query is being run via specific sql for performance reasons as the data set is expected to be sufficiently large, frequently changing (cache may not help all that much) and frequently asked.

Answer 1

If I understand you correctly, you want to present a full range of YYYY-WW labels and counts, starting with start_date regardless of whether there's data for a given week. You didn't mention if previsto is a Date, a Time, or a DateTime; I'll assume it's a Time just for maximum inconvenience. =]
I think the main challenge you're struggling with is that you're conflating the dataset with the presentation. I like handling the two separately: first, get the data and put it into a year-week format; then present the range.
For getting the data, I like a more Railsy, less database-specific solution. Your mileage may vary, especially if you've got a large dataset and/or need to make the database do the heavy lifting. Here's a query that gets only the previsto field for each record while also forcing the database to evaluate the date range. Probably the most concise query without having to break out SQL:
#interventos_2 = Intervento.select(:previsto).
where(previsto: (start_date..Time.now)).
map {|iv| iv.previsto.strftime('%Y-%W')}
Note that this also maps the result down to a simple array of YYYY-WW. Speaking of which, let's map out that range of YYYY-WW now:
# make sure the end_date a clean multiple of 7 days from start_date
end_date = Date.today + (7 - (Date.today - start_date.to_date) % 7)
#timespan = (start_date.to_date..end_date).step(7).map {|date| date.strftime('%Y-%W')}
(There are probably much tidier ways to write that)
Given those bits, here's a version of your view code that presents the full range of weeks and the count for that week, even if it's 0:
<% #timespan.each do |yearweek| %>
<%= yearweek %> <%= #interventos_2.count(yearweek) %><br />
<% end %>
In bocca al lupo!
Update: Your update notes that your use case requires the direct SQL query, so here's the same general approach with that in mind:
#interventos_2 = Intervento.where(['previsto >= ?', start_date]).
group("DATE_TRUNC('week', previsto)").count.
map {|timestamp,count| [timestamp.strftime('%Y-%W'), count]}.to_h
# make sure the end_date a clean multiple of 7 days from start_date
end_date = Date.today + (7 - (Date.today - start_date.to_date) % 7)
#timespan = (start_date.to_date..end_date).step(7).map {|date| date.strftime('%Y-%W')}
<% #timespan.each do |yearweek| %>
<%= yearweek %> <%= #interventos_2[yearweek] || 0 %><br />
<% end %>

Answer 2

With the following controller method, the range of weeks and the data required for the view is generated (beginning_of_week for comparing with the has data, in addition to the data itself).
#weeks = []
while start_date < final_date
#weeks[start_date.year] = [] unless #weeks[start_date.year]
#weeks << [start_date.beginning_of_week, start_date.cweek]
start_date += 1.week
end
#weeks.reject!{|a| a.blank?}
Thus the view can generate a layout item for each week and compare it to the hash, and where there is nil, generate zero.
<% #weeks.each do |week| %>
<% if !week.nil? %>
<%= week[0] %>:
<% z = #interventos_2.detect {|f| f[0] == week[0] } %>
<% if !z.nil? %>
<%= z.to_a[1] %>
<% else %>
0
<% end %>
<% end %>