Rails: Group results by week (using group_by)
The Enumerable class in Rails contains a method named ‘group_by’. This method is pure magic for a developer’s point of view. I’ll give you a simple example that shows the power of group_by.
Let’s say you have a table ‘posts’ containing blog posts. Now, you normally show these chronologically a few at a time. Nothing special there. For some special overview page, you want to group your posts by week.
With normal ActiveRecord operations this would be quite an elaborate task. But with group_by from Enumerable, it becomes child’s play.
First of all, in the controller, just get all the posts you need. In this case, all of them:
Controller:
def list @posts = Post.find :all end
As you can see, I perform no ordering or whatsoever here.
Now, in your view you normally would iterate over all posts like this:
< %= render :partial => 'post', :collection => @posts %>
But, as I said, we want to group the posts by week. To make life easy, I add a method to the Post class that returns the week number in which a post was written:
Model Post:
def week self.created_at.strftime('%W') end
Now, the magic will happen in our view:
< % @posts.group_by(&:week).each do |week, posts| %>
<div id="week">
<h2>Week < %= week %></h2>
< %= render :partial => 'post', :collection => @posts %>
</div>
< % end %>Let me explain the above. We specify that we want to call group_by for @posts. But we need to say how we want to group these posts. By specifying &:week we tell group_by that we want to group by the result of the week attribute of every post. This is the attribute we specified earlier in the model.
Well, when the grouping is done we create a block that will handle every group of items. We extract ‘week’ and ‘posts’ here. ‘week’ contains the week number and ‘posts’ all the posts for that week.
As normal, we can now show the week number and iterate over the posts.
Sorting groups
The result of group_by is not guaranteed to be ordered in any way. Simply call ’sort’ before each and you’re set:
@posts.group_by(&:week).sort.each do |week, posts|
Mostly, you’ll find that the posts for every group are not sorted either. With the example above I think it’s easy to figure out how to do that now. (hint: .sort)
Great post!!! This was exactly what I was looking for. Thank you!
Wouldn’t it be better to offload that work to the database, though?
How exactly would you sort the posts inside the sorted weeks? I am doing something similar in my app but can’t figure out how to sort the records inside the group.
@juco: You could do it like this:
render :partial => ‘post’, :collection => @posts.sort {|a,b| a.title < => b.title}
See Ruby’s sort documentation on how to sort your @posts.
So, I can get the groups listed separately… Now let’s say I want to re-render only one of the partials (say by column header sorting on a diff property). Is this possible? How would you go about it?
@skwasha: You could give every week a seperate id: instead of <div id=”week”>, you’d use <div id=”week-<%= week %>”>
When you want to update your group, just select the appropriate data from you database (using conditions on your model like find_all_for_week or something. And then, using RJS, replace the div:
render :update do |page|
page.replace_html “week-#{week}”, :partial => ‘your group partial’
end
[...] Rails: Group results by week – This came in handy last night when I needed to do multilevel reporting in a Rails view. [...]
Hello,
What goes in side the partial? When I wrote this in rails 2.0.2, it said that it couldn’t find the _post.html.erb partial to render. I created that and placed some queries inside but it just posted everything once for each week.
Thanks!
I’m having the same issue, I get it broken up nicely by week, but each post is listed under each week. ie 3 weeks = triple the posts.
Ah ha! I found that (for me at least) this:
Week
‘post’, :collection => @posts %>
should read:
Week
‘post’, :collection => posts %>
*the @ symbol should be removed from ‘:collection => @posts’
This solved my problem of getting repeating posts. It seems that the collection should be of the local variable posts rather than the instance variable of @post (which contains all of the entries in the database)
How about if you want to group_by one attribute but you want to sort the resultant groups by another attribute of the object?
i.e., I am grouping by the distance_of_time_in_words value of the release_date attribute but when I sort the resultant hash, everything’s sorted alphabetically (i.e. instead of “6 Days “, “5 Weeks”, “1 Month”, “4 Months” I get “1 Month”, “4 Months”, “5 Weeks”, “6 Days”)
Here’s some of my code:
Controller:
@items = Category.all_category_items.group_by {|i| distance_of_time_in_words(Time.now, i.release_date)}
View:
This gives me the alphabetical order.
How do I sort by the first @item.release_date in the group instead of the distance_of_time_in_words(Time.now, i.release_date) but still group by distance_of_time_in_words(Time.now, i.release_date)?
Whoops, forgot to add the view code:
@items.keys.sort.each do |release_date|
Thank you, man! eventize.de now groups events. Cheers!
Great post! Only problem is your color scheme. Purple on gray is not readable.