# Simple question about collections

**URL:** <https://www.ruby-forum.com/t/simple-question-about-collections/174555>\
**Category:** Rails\
**Created:** [August 28, 2009, 4:01pm UTC](https://www.ruby-forum.com/t/simple-question-about-collections/174555 "2009-08-28T16:01:03Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![trevoke](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ee7513/32.png) [@trevoke](https://www.ruby-forum.com/u/trevoke)\
**Post date:** [August 28, 2009, 4:01pm UTC](https://www.ruby-forum.com/t/simple-question-about-collections/174555/1 "2009-08-28T16:01:03Z")

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Alright, so… Let’s try this:

1. I have a view which one way or another calls a controller action and  
wants to update a div.

2. This controller action finds a collection of records and does a  
render :partial with said collection  
(e.g. : @patients with Patient.last and Patient.first)

3. … What’s the partial going to look like? This is where I don’t  
understand how to use collections.

If I do a ":collection =\> " call, what exactly am I asking Rails to do…  
And then how do I help it do that?

---

<div class="post-metadata">

**Author:** ![trevoke](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ee7513/32.png) [@trevoke](https://www.ruby-forum.com/u/trevoke)\
**Post date:** [August 30, 2009, 3:36pm UTC](https://www.ruby-forum.com/t/simple-question-about-collections/174555/2 "2009-08-30T15:36:25Z")

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> If I do a ":collection =\> " call, what exactly am I asking Rails to do…

You are enabling a local variable in your partial with the same name  
as the partial. The variable name will be singular and will be  
initialized to each of the values that :collection contains.

Here’s an example:

* * *

from: photos\_controller.rb

def index  
@photos = Photo.paginate(:page =\> params[:page],  
:conditions =\> ‘thumbnail IS NULL’,  
:order =\> ‘created\_at’,  
:per\_page =\> 9  
)  
respond\_to do |format|  
format.html # index.html.erb  
format.xml { render :xml =\> @photos.to\_xml }  
end  
end

* * *

from: views/photos/index.html.erb

## All Photos

\<% params[:fromURL] = photos\_path() %\>

\<%= will\_paginate @photos %\>

 \<%= render :partial =\> 'photo', :collection =\> @photos %\>

\<%= will\_paginate @photos %\>

* * *

from: views/photos/\_photo.html.erb

- \<%= link\_to image\_tag(photo.public\_filename('thumb')), photo\_path(:id =\> photo) %\>

This sequence has the result of presenting a paginated view with nine  
thumbnails, each of which is linked to the full sized image it  
thumbs. The pagination will allow for the presentation of all  
thumbnails in the collection.

So the sequence is:

1. controller identifies complete collection as paginated and passes  
in to view as @photos
2. view deals with pagination action of :per\_page photos and passes  
each photo in turn to the partial
3. partial presents each photo as its’ thumbnail linked to its’ full  
sized variant

This is just one of several ways to use partials but I think you  
should be able to understand. One big gain here is the absence of the  
for loop needed to do the nine thumbnails. The other is the ability  
to call the partial from another controller/view sequence, i.e. here  
we’re stepping through all the photos in the library by, as the photo  
model is associated with a user model, we can also use the partial to  
view all the photos owned by a specific user.

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<div class="post-metadata">

**Author:** ![trevoke](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/t/ee7513/32.png) [@trevoke](https://www.ruby-forum.com/u/trevoke)\
**Post date:** [August 31, 2009, 2:50pm UTC](https://www.ruby-forum.com/t/simple-question-about-collections/174555/3 "2009-08-31T14:50:43Z")

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I see. Thank you!