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value_changed? helper method

#2281

Added a new helper method to TextHelper called value_changed?, also includes test cases and documentation.

value_changed? tracks the value passed in and returns true whenever the value changes from its previous setting. It can be useful when iterating through a set of data that is organized in subsets to display a new title for each subset.


# Assuming people has been sorted by occupation...
<% @people.each do |person| %>
  <%= "<h2>#{person.occupation.pluralize}</h2>" if value_changed?(person.occupation) %>
  <%= person.name %><br/>
<% end %>

This produces:


<h2>Doctors</h2>
Jane Doe
John Smith
<h2>Programmers</h2>
Rails Rules

This little bit of code replaces the common pattern of:


<% previous_value = nil %>
<% @people.each do |person| %>
  <% if previous_value != person.occupation %>
    <% previous_value = person.occupation %>
    <h2><%= person.occupation.pluralize %></h2>
  <% end %>
  <%= person.name %><br/>
<% end %>

Also, value_changed? accepts a name parameter to support nesting in loops (by default, the value is named "default").

I hope this is useful for others. It sure has been handy in my projects. Thanks!

value_changed_helper_method.diff 3.02 KB

Reported by Jason Arora · March 17th, 2009 @ 11:21 PM

Activity

Ian Terrell

Are there other use cases?

Do you see this as much better than using Enumerable#group_by? (http://api.rubyonrails.org/class...)


<% @people.group_by(&:occupation).each do |occupation, peeps| %>
  <h2><%= occupation %></h2>
  <% peeps.each do |person| %>
    <%= person.name %>
  <% end %>
<% end %>

March 18th, 2009 @ 04:35 PM

Jason Arora
Thanks for posting, Ian. You are correct, Enumerable#group_by is the best way to handle this pattern when ordering by a model attribute.

However, the value_changed? method works really well for dynamic cases, such as:
```
<% @people.each do |person| %>
  <%= "<h2>#{person.name.first}</h2>" if value_changed?(person.name.first) %>
  <%= person.name %><br/>
<% end %>

# => <h2>A</h2>
# => Alan
# => Amy
# => <h2>B</h2>
# => Barry
# => ...
```
Is this more acceptable? Thanks again for the feedback.

Regards, Jason
March 21st, 2009 @ 02:01 AM
Ian Terrell
For this use case, Enumerable#group_by still seems best. The &:symbol syntax is shorthand for a block; you can spell it out for cases where you wish to group by something that isn't a method:
```
<% @people.group_by{|person|person.name.first}.each do |initial, peeps| %>
  <h2><%= initial %></h2>
  <% peeps.each do |person| %>
    <%= person.name %>
  <% end %>
<% end %>
```
March 21st, 2009 @ 08:12 PM
Jeremy Kemper
- State changed from new to wontfix
Nice patch, but agreed that group_by is a better choice for the problem it's solving

March 22nd, 2009 @ 01:42 AM
Jason Arora
Duh. :-P

You're totally right, Ian. Thanks!

March 22nd, 2009 @ 05:55 AM