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Rails’ attachment_fu, :thumbnail_class and you

I was having a bit of trouble with attachment_fu that took a while to figure out, so I thought I’d post my solution for the next person.

I have a Photo model that I’m using to store pictures. Since attachment_fu will automatically resize, create thumbnails, and store pictures on the file system, it was an easy choice to use it. The things I found I didn’t like about it:

It stores thumbnails as separate records inside the photos table. This means you have to check if thumbnail.nil? each time you display the pictures, and you’ll have to check parent_id.nil? to count your photos.
You can’t use your own model validations. For example, I wanted to use

validates_presence_of :name
validates_presence_of :description

But I’d always get validation errors because attachment_fu tries to save the thumbnail attributes when it creates or resizes a thumbnail. As you can imagine, this is a major problem.

:thumbnail_class to the rescue

Mike Clark’s attachment_fu blog post mentions that you can use the :thumbnail_class argument to separate your model validations and attachment_fu’s validations. Here’s how to do it:

class Photo < ActiveRecord::Base
  has_many :thumbnails, :foreign_key => 'parent_id'
 
  has_attachment  :storage => :file_system,
                          :content_type => :image,
                          :max_size => 10.megabytes,
                          :resize_to => '640x480',
                          :thumbnails => { :thumb => '100x100' },
                          :thumbnail_class => Thumbnail
 
  # Validations
  validates_presence_of :name
  validates_presence_of :description
end
 
class Thumbnail < ActiveRecord::Base
  belongs_to :photo, :foreign_key => 'parent_id'
 
  has_attachment  :storage => :file_system,
                  :content_type => :image
end

The killer for me initially was that I wasn’t specifying has_attachment in the Thumbnail model. I always got this error:

undefined method `temp_path=' for #thumbnail:0xb69a9514

So save yourself by putting has_attachment in both models. Make sure to define any attachment_fu arguments in your Photo model, and leave the Thumbnail model bare. You’ll also want to make sure you have the normal attachment_fu schema in both models:

      t.column :parent_id,  :integer
      t.column :content_type, :string
      t.column :filename, :string
      t.column :thumbnail, :string
      t.column :size, :integer
      t.column :width, :integer
      t.column :height, :integer

I’ve found that everything seems to be working as normal going this route. My model validations work and the thumbnails are not polluting the photos table. If you want to find the Photo for a particular Thumbnail, keep in mind that parent_id now refers to the id in the Photo model:

t = Thumbnail.find ....  # find your thumbnail
p = Photo.find(t.parent_id)

And for the ultimate ease-of-use relationship, just use:

t = Thumbnail.find ....  # find your thumbnail
p = t.photo               # get a photo
p.thumbnails              # get all thumbnails

This works since we defined the has_many relationship in the model.

Let me know if you have any problems with this method or if it helped you!

2007 11 19

Comments (6)

How to create a lookup table in Ruby on Rails

I’m a big fan of the Rails way, but sometimes the simple things get you. I like to set up “lookup tables” in Rails, ie. tables in the database that hold commonly used, (often) fixed values. For example, if I have a Car model, I might want to define a Car type model. This would have two purposes:

Give me the ability to associate a type with the Car model (ie. Car.type => ’sport’)
Pre-load common values into the database (ie. sport, coupe, sedan, etc)

2007 10 18

Rails’ ruby script/console has tab completion!

While I recently found out about Ruby’s ‘methods’ method, I also found that Rails’ “ruby script/console” will do tab completion:

>> a = Array.new
=> []
>> a.
Display all 157 possibilities? (y or n)
a.fetch                              a.method                             a.slice
a.__id__                             a.fill                               a.methods                            a.slice!
a.__send__                           a.find                               a.min                                a.sort
a.all?                               a.find_all                           a.nil?                               a.sort!
a.any?                               a.first                              a.nitems                             a.sort_by
a.assoc                              a.flatten                            a.object_id                          a.split
a.at                                 a.flatten!                           a.pack                               a.subclasses_of
a.b64encode                          a.freeze                             a.partition                          a.sum
a.blank?                             a.frozen?                            a.pop                                a.suppress
a.class                              a.gem                                a.pretty_inspect                     a.taguri
a.clear                              a.grep                               a.pretty_print                       a.taguri=
a.clone                              a.group_by                           a.pretty_print_cycle                 a.taint
a.collect                            a.hash                               a.pretty_print_inspect               a.tainted?
a.collect!                           a.id                                 a.pretty_print_instance_variables    a.to_a
a.compact                            a.in_groups_of                       a.private_methods                    a.to_ary
a.compact!                           a.include?                           a.protected_methods                  a.to_default_s
a.concat                             a.index                              a.public_methods                     a.to_formatted_s
a.copy_instance_variables_from       a.index_by                           a.push                               a.to_json
a.daemonize                          a.indexes                            a.rassoc                             a.to_param
a.dclone                             a.indices                            a.reject                             a.to_s
a.decode64                           a.inject                             a.reject!                            a.to_sentence
a.decode_b                           a.insert                             a.remove_subclasses_of               a.to_set
a.delete                             a.inspect                            a.replace                            a.to_xml
a.delete_at                          a.instance_eval                      a.require                            a.to_yaml
a.delete_if                          a.instance_exec                      a.require_gem                        a.to_yaml_properties
a.detect                             a.instance_of?                       a.require_library_or_gem             a.to_yaml_style
a.display                            a.instance_values                    a.respond_to?                        a.transpose
a.dup                                a.instance_variable_get              a.returning                          a.type
a.each                               a.instance_variable_set              a.reverse                            a.uniq
a.each_index                         a.instance_variables                 a.reverse!                           a.uniq!
a.each_with_index                    a.is_a?                              a.reverse_each                       a.unloadable
a.empty?                             a.join                               a.rindex                             a.unshift
a.enable_warnings                    a.kind_of?                           a.select                             a.untaint
a.encode64                           a.last                               a.send                               a.values_at
a.entries                            a.length                             a.shift                              a.with_options
a.eql?                               a.load                               a.silence_stderr                     a.yaml_initialize
a.equal?                             a.map                                a.silence_stream                     a.zip
a.extend                             a.map!                               a.silence_warnings
a.extend_with_included_modules_from  a.max                                a.singleton_methods
a.extended_by                        a.member?                            a.size

Unfortunately the same thing doesn’t work for the irb. Of course you can always

a.methods.sort

to see the same information.

2007 10 12

Installing Mongrel on Ubuntu Feisty with rubygems

When I recently re-installed Ubuntu 7.04 (Feisty), I found that I couldn’t build the native fastthread gem (a required dependency). If you get this error:
Building native extensions. This could take a while... extconf.rb:1:in `require': no such file to load -- mkmf (LoadError) from extconf.rb:1
ERROR: While executing gem ... (RuntimeError) ERROR: Failed to build gem native extension. Gem files will remain installed in /var/lib/gems/1.8/gems/fastthread-1.0 for inspection.
then you need the ruby development libraries and the build-essential package. I’ve found that these commands will make sure mongrel (and rails) is installed correctly:
$ sudo apt-get install ruby1.8-dev rubygems build-essential rails $ sudo gem install mongrel --include-dependencies

2007 10 03

linux
rails
ruby

Ruby has a “methods” method to see all available methods

So it turns out that Ruby has a few methods that can really help you figure out what you’re doing when you’re working with sparsely documented APIs or you just want a quick reference.

I’ve used the “inspect” method for a long time. You can basically see all of the class members and sometimes other useful information. For example:
irb(main):001:0> s = "My String" => "My String" irb(main):002:0> s.inspect => "\"My String\""
irb(main):003:0> a = [1, 2, 3] => [1, 2, 3] irb(main):004:0> a.inspect => “[1, 2, 3]”

The inspect method can give you a good idea of what data is in a particular object. But what if you want to see all methods an object holds? Well duh…. use #methods.

irb(main):001:0> require 'pp' => true irb(main):002:0> s = "My String" => "My String" irb(main):003:0> pp s.methods.sort ["%", "*", "+", "<", "<<", "<=", "<=>", "==", "===", "=~", ">", ">=", "[]“, “[]=”, “__id__”, “__send__”, “all?”, “any?”, “between?”, “capitalize”, “capitalize!”, “casecmp”, “center”, “chomp”, “chomp!”, “chop”, “chop!”, “class”, “clone”, “collect”, “concat”, “count”, “crypt”, “delete”, “delete!”, “detect”, “display”, “downcase”, “downcase!”, “dump”, “dup”, “each”, “each_byte”, “each_line”, “each_with_index”, “empty?”, “entries”, “eql?”, “equal?”, “extend”, “find”, “find_all”, “freeze”, “frozen?”, “grep”, “gsub”, “gsub!”, “hash”, “hex”, “id”, “include?”, “index”, “inject”, “insert”, “inspect”, “instance_eval”, “instance_of?”, “instance_variable_get”, “instance_variable_set”, “instance_variables”, “intern”, “is_a?”, “kind_of?”, “length”, “ljust”, “lstrip”, “lstrip!”, “map”, “match”, “max”, “member?”, “method”, “methods”, “min”, “next”, “next!”, “nil?”, “object_id”, “oct”, “partition”, “pretty_inspect”, “pretty_print”, “pretty_print_cycle”, “pretty_print_inspect”, “pretty_print_instance_variables”, “private_methods”, “protected_methods”, “public_methods”, “reject”, “replace”, “respond_to?”, “reverse”, “reverse!”, “rindex”, “rjust”, “rstrip”, “rstrip!”, “scan”, “select”, “send”, “singleton_methods”, “size”, “slice”, “slice!”, “sort”, “sort_by”, “split”, “squeeze”, “squeeze!”, “strip”, “strip!”, “sub”, “sub!”, “succ”, “succ!”, “sum”, “swapcase”, “swapcase!”, “taint”, “tainted?”, “to_a”, “to_f”, “to_i”, “to_s”, “to_str”, “to_sym”, “tr”, “tr!”, “tr_s”, “tr_s!”, “type”, “unpack”, “untaint”, “upcase”, “upcase!”, “upto”, “zip”] => nil

Useful, huh?

2007 09 28

ruby