Dynamically display table based on conditions - ruby-on-rails

Question

Hello my fellow programmers, I am a ruby on rails noob and need some help:
Based on today's date I am querying the db and getting the results as an array.
#cr = [#<CRate id: 1, currency: "AUD", rate: 2.0, datetime: "2013-10-09 22:59:59">,
#<CRate id: 7, currency: "USD", rate: 10.0, datetime: "2013-10-09 29:50:50">,
#<CRate id: 9, currency: "EUR", rate: 20.0, datetime: "2013-10-09 22:59:59">,
#<CRate id: 12, currency: "RUB", rate: 12.0, datetime: "2013-10-09 22:59:59">,
#<CRate id: 14, currency: "AUD", rate: 18.0, datetime: "2013-10-09 29:50:50">]
Question: I need to dynamically display the table based on currency(header), depending on the time, if rate exist then I display it else output is nil. Like this table below:
I have tried to do this:
<table>
<thead>
<th>Time</th>
<% #currency = #cr.collect(&:currency) %>
<% #currency.each do |cur| %>
<th><%= cur %></th>
<% end %>
</thead>
<tbody>
<tr class="<%= cycle('odd', 'even') %>">
<td><%= #datetime = #cr.collect(&:datetime) %></td>
<% #rate = #cr.collect(&:rate) %>
<% #rate.each do |r| %>
<td><%= r %></td>
<% end %>
</tr>
</tbody>
</table>
This doesn't work because the thead is updated with 2 AUD's and I don't know how to proceed. Please help.
Thanks a lot.

Answer 1

First, you need unique currency types, so line no. 4 will be:
<% #currency = #cr.collect(&:currency).uniq %>
Secondly, you need to get rates based on datetime and currency type (since you have not specified Rails version, I will try and write a working solution which might not be best solution):
<tbody>
<% #datetime = #cr.collect(&:datetime).uniq %>
<% #datetime.each do |dt| %>
<tr class="<%= cycle('odd', 'even') %>">
<td><%= dt %></td>
<% #currency.each do |curr| %>
<td><%= #cr.select {|o| o.currency == curr && o.datetime == dt}.rate %></td>
<% end %>
</tr>
<% end %>
</tbody>
HTH
Edit: This seems to be an inefficient way to solve this problem because you are relying too much on arrays in a way that they will always come in order you are expecting. I would like rather create a Hash (better known as key-value pair) of #cr like {'first_datetime' => {'AUD' => '10', 'RUB' => '20'}, 'second_datetime' => {'AUD' => '50', 'RUB' => ''}} and so forth.
Edit 2: In your controller, create a hash and 2 sets like:
#currencies = Set.new
#datetimes = Set.new
#currency_rate_datewise = Hash.new {|h, k| h[k] = {}}
#cr.each do |cr|
#currencies.add cr.currency
#datetimes.add cr.datetime
#currency_rate_datewise[cr.datetime][cr.currency] = cr.rate
end
Now your view could be simplified to:
<table>
<thead>
<th>Time</th>
<% #currencies.each do |cur| %>
<th><%= cur %></th>
<% end %>
</thead>
<tbody>
<% #datetimes.each do |datetime| %>
<tr class="<%= cycle('odd', 'even') %>">
<td><%= datetime %></td>
<% #currencies.each do |currency| %>
<td><%= #currency_rate_datewise[datetime][currency] || 'nil' %></td>
<% end %>
</tr>
<% end %>
</tbody>
</table>
Although, I would have used group_by selectively, but I guess, it is better if you stick with learning such things at beginner's stage. I have written this from top of my head, please post a comment if something's wrong.
HTH

Answer 2

you can use the group_by rails helper for that. see the documentation:
group_by() Collect an enumerable into sets, grouped by the result of a
block. Useful, for example, for grouping records by date.
Example:
latest_transcripts.group_by(&:day).each do |day, transcripts|
p "#{day} -> #{transcripts.map(&:class).join(', ')}"
end
"2006-03-01 -> Transcript"
"2006-02-28 -> Transcript"
"2006-02-27 -> Transcript, Transcript"
"2006-02-26 -> Transcript, Transcript"
"2006-02-25 -> Transcript"
"2006-02-24 -> Transcript, Transcript"
"2006-02-23 -> Transcript"