Ruby / Rails - Advanced grouping with group_by - ruby

Question

I have a model called event_recurrences it contains 2 important columns event_id and cached_schedule
cached_schedule contains an array of dates
event_id is used to reference the event
I need to
Get all the event_recurrence objects #something.event_recurrences - Done
Go through each recurrence object and get the event_id and all the
dates from cached_schedule
Iterate through each month, and spit out a list like the following
Jan
event_id date
event_id date
event_id date
Feb
event_id date
... and so on
To recap the event_id is located event_recurrence.event_id the dates that the event_id will happen on are located in an array inside event_recurrence.cached_schedule
Some I have some incomplete code to work with...
This code works successfully to show each event_recurrence object by month using the created_at field.
in my controller
#schedule_months = #something.event_recurrences.order("created_at DESC").group_by { |e| e.created_at.beginning_of_month }
in my view
<% #schedule_months.keys.sort.each do |month| %>
<div class="month">
<%= month.strftime("%B %Y") %>
</div>
<% for event in #schedule_months[month] %>
<li><%= event %></li>
<% end %>
<% end %>
Thanks in advance.
Sincerely, jBeas

Answer 1

There are some details missing from your question... for example, can the dates in cached_schedule span multiple months, or are they all guaranteed to be in the same month?
If you just want to use core Ruby:
dates = []
#something.event_recurrences.each do |er|
er.cached_schedule.each { |date| dates << [date, er.event_id] }
end
#event_dates = dates.group_by { |(date,event_id)| date.mon }
Then in the view:
<% #event_dates.keys.sort.each do |month| %>
<div class="month"> <%= month.strftime("%B %Y") %></div>
<% #event_dates[month].each do |(date,event_id)| %>
<li><%= date %> <%= event_id %></li>
<% end %>
<% end %>
You may have to adjust the code a little depending on the specifics, but this should give you the idea. Note the use of destructuring assignment: #event_dates[month].each do |(date,event_id)|. This saves a line of code and expresses what the code is doing more clearly.
If you don't mind adding your own extensions to the core Ruby classes, you could make this code even cleaner and more consise. I often use a method which I call mappend:
module Enumerable
def mappend
result = []
each { |a| enum = yield a; enum.each { |b| result << b } if enum }
result
end
end
The name is a mix of map and append -- it is like map, but it expects the return value of the mapping block to also be Enumerable, and it "appends" all the returned Enumerables into a single Array. With this, you could write:
#event_dates = #something.event_recurrences.mappend { |er| er.cached_schedule.map { |date| [date, er.event_id] }}.group_by { |(date,event_id)| date.mon }
OK, that might be a lot for one line, but you get the idea: it saves you from using an intermediate variable to accumulate results.
UPDATE: Something like mappend is now part of the Ruby core library! It's called flat_map.