Rails%203%20In%20Action

216 CHAPTER 9 File uploading
that the form is multipart anyway, so you should do this now by changing the
form_for line in app/views/tickets/_form.html.erb from this
to this:
{ :multipart => true } do |f| %>
Now we come to a very interesting point in implementing file uploading. When you
run bin/cucumber features/creating_tickets.feature, all of the scenarios are
broken and all for the same reason:
And I press "Create Ticket"
unknown attribute: asset (ActiveRecord::UnknownAttributeError)
Because you added this file_field, the create action’s code dutifully tries to assign
it as an attribute, only to find that it’s not defined and so causes this error. Rather than
running a migration to add an attribute by this name, you use the Paperclip gem to
handle it.
9.1.2 Enter stage right, Paperclip
Just as you would use a normal paperclip to attach paper files together, in your application
you use the Paperclip gem to provide the attachment functionality you need
for tickets. This gem was created by thoughtbot, 4 which has a whole slew of other useful
gems, such as Hoptoad. 5
To install Paperclip, you need to add a line to the Gemfile to tell Bundler that you
want to use this gem. Put this underneath the line specifying the CanCan gem, separated
by a line because it’s a different type of gem (CanCan has to do with users,
paperclip has to do with files):
gem 'cancan'
gem 'paperclip'
Next, you must run bundle install to install this gem.
With Paperclip now installed, you can work on defining the asset attribute that
your model wants. It’s not really an attribute; the error message is misleading in that
respect. All it needs is a setter method (asset=) and it would be happy. But you need
this method to do more than set an attribute on this object; you need it to accept the
uploaded file and store it locally. Paperclip lets you define this fairly easily with its
has_attached_file method. This method goes in the Ticket model, defines the setter
method you need, and gives four application the ability to accept and process this
file. Add it to your Ticket model with this line:
has_attached_file :asset
Now this asset= method is defined, but it’s not yet over!
4 http://thoughtbot.com.
5 For a full list of thoughtbot’s gems, see its GitHub page: http://github.com/thoughtbot.