Ruby on Rails sort on the result of a method

Question

I have a helper method in a Rails site that checks how many attendees are in a certain class. I have two different entities here, one for the class and one for attendee. The helper method takes an activerecord result as an argument and calculates how many open slots there are based on the total spots minus the total people that have registered. Is there any way that I can sort by the result of this so that courses with no open spots are put at the end of the list? Don't know if it will affect anything, but I am also using the will_paginate gem on the result set as well. I am currently ordering by the start date.

Method in math_class.helper

def open_slots(math_class)
  math_attendees = Attendee.where(:math_class_id => math_class.id).count
  return math_class.total_spots - math_attendees
end

Math Class View/Open Slots Column

<% @math_classes.each do |math_class| %>
   <!-- Other columns... -->
<% if open_slots(math_class) > 0 %>
  <td>
   <%= pluralize(open_slots(math_class), 'slot') %> of 
   <%= math_class.total_spots %> remaining
   </td>
<% else %>
   <td><span class="text-error">No Open Slots</span></td>
<% end %>

Controller Query statement

@math_classes = MathClass.joins(:room).order("starts_at").page(params[:page]).per_page(100)

Nick Aschenbach · Accepted Answer · 2015-12-29 18:52:36Z

1

Consider using the block form of Array#sort:

 @math_classes = MathClass.joins(:room).order("starts_at").page(params[:page]).per_page(100)
 @math_classes.to_a.sort! do |a, b|
   a_open_spots = a.total_spots - a.attendees.count
   b_open_spots = b.total_spots - b.attendees.count
   a_open_spots <=> b_open_spots
 end

The spaceship operator <=> returns -1, 0 or 1 depending on if the left hand side is less than, equal to or greater than the right side. For example:

 3 <=> 4 # => -1
 3 <=> 3 # => 0
 4 <=> 3 # => 1

Array#sort uses this to order the elements in the array.

edited Dec 29, 2015 at 18:52

answered Dec 29, 2015 at 17:22

Nick Aschenbach

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2 Comments

SausageBuscuit Over a year ago

Seems handy, but total_spots is a column within math_classes. I get an error when I attempt this that total_spots is undefined. If you attempt a to_a conversion wouldn't that remove the activerecord column names and not allow you to reference total_spots?

Nick Aschenbach Over a year ago

The instance variable @math_classes should be something like MathClass::ActiveRecord_Relation. The to_a converts this relation into actual instances of the MathClass. From there, we should be able to sort based on attributes of this class. For example: math_classes.to_a.sort { |a, b| a.total_spots <=> b.total_spots }

baron816 · Accepted Answer · 2015-12-29 15:50:19Z

-1

You're going to have to use the order_by Ruby method.

answered Dec 29, 2015 at 15:50

baron816

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