Run this command:

ruby script/generate model State

And put this in db/migrate/XXX_create_states.rb:

class CreateStates < ActiveRecord::Migration
  def self.up
    create_table :states do |t|
      t.column :name, :string
      t.column :abbreviation, :string
    end
 
    State.create :name => 'Alabama', :abbreviation => 'AL'
    State.create :name => 'Alaska', :abbreviation => 'AK'
    State.create :name => 'Arizona', :abbreviation => 'AZ'
    State.create :name => 'Arkansas', :abbreviation => 'AR'
    State.create :name => 'California', :abbreviation => 'CA'
    State.create :name => 'Colorado', :abbreviation => 'CO'
    State.create :name => 'Connecticut', :abbreviation => 'CT'
    State.create :name => 'Delaware', :abbreviation => 'DE'
    State.create :name => 'District of Columbia', :abbreviation => 'DC'
    State.create :name => 'Florida', :abbreviation => 'FL'
    State.create :name => 'Georgia', :abbreviation => 'GA'
    State.create :name => 'Hawaii', :abbreviation => 'HI'
    State.create :name => 'Idaho', :abbreviation => 'ID'
    State.create :name => 'Illinois', :abbreviation => 'IL'
    State.create :name => 'Indiana', :abbreviation => 'IN'
    State.create :name => 'Iowa', :abbreviation => 'IA'
    State.create :name => 'Kansas', :abbreviation => 'KS'
    State.create :name => 'Kentucky', :abbreviation => 'KY'
    State.create :name => 'Louisiana', :abbreviation => 'LA'
    State.create :name => 'Maine', :abbreviation => 'ME'
    State.create :name => 'Maryland', :abbreviation => 'MD'
    State.create :name => 'Massachutsetts', :abbreviation => 'MA'
    State.create :name => 'Michigan', :abbreviation => 'MI'
    State.create :name => 'Minnesota', :abbreviation => 'MN'
    State.create :name => 'Mississippi', :abbreviation => 'MS'
    State.create :name => 'Missouri', :abbreviation => 'MO'
    State.create :name => 'Montana', :abbreviation => 'MT'
    State.create :name => 'Nebraska', :abbreviation => 'NE'
    State.create :name => 'Nevada', :abbreviation => 'NV'
    State.create :name => 'New Hampshire', :abbreviation => 'NH'
    State.create :name => 'New Jersey', :abbreviation => 'NJ'
    State.create :name => 'New Mexico', :abbreviation => 'NM'
    State.create :name => 'New York', :abbreviation => 'NY'
    State.create :name => 'North Carolina', :abbreviation => 'NC'
    State.create :name => 'North Dakota', :abbreviation => 'ND'
    State.create :name => 'Ohio', :abbreviation => 'OH'
    State.create :name => 'Oklahoma', :abbreviation => 'OK'
    State.create :name => 'Oregon', :abbreviation => 'OR'
    State.create :name => 'Pennsylvania', :abbreviation => 'PA'
    State.create :name => 'Rhode Island', :abbreviation => 'RI'
    State.create :name => 'South Carolina', :abbreviation => 'SC'
    State.create :name => 'South Dakota', :abbreviation => 'SD'
    State.create :name => 'Tennessee', :abbreviation => 'TN'
    State.create :name => 'Texas', :abbreviation => 'TX'
    State.create :name => 'Utah', :abbreviation => 'UT'
    State.create :name => 'Vermont', :abbreviation => 'VT'
    State.create :name => 'Virginia', :abbreviation => 'VA'
    State.create :name => 'Washington', :abbreviation => 'WA'
    State.create :name => 'West Virginia', :abbreviation => 'WV'
    State.create :name => 'Wisconsin', :abbreviation => 'WI'
    State.create :name => 'Wyoming', :abbreviation => 'WY'
 
  end
 
  def self.down
    drop_table :states
  end
end

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:

1. 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.
2. 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!

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

I can accomplish this through a few steps. First, lets generate the lookup table model:

$ ruby script/generate model CarType

We’ll need to edit the migration under db/migrate/XXX_create_car_types.rb

class CreateCarTypes < ActiveRecord::Migration
  def self.up
    create_table :car_types do |t|
      t.column :name, :string
    end
 
    CarType.create(:name => 'sedan')
    CarType.create(:name => 'sport')
    CarType.create(:name => 'coupe')
    CarType.create(:name => 'truck')
    CarType.create(:name => 'van')
 
  end
 
  def self.down
    drop_table :car_types
  end
end

You’ll note that I created the name field, and then I just used the create method to produce sample values in the database. This can be an immensely useful technique, especially when deploying a production website – just run your migrations and those lookup tables are already populated.

Note: The other technique for populating the database automatically is fixtures. While I think they are great for creating test development data, fixtures fall behind in a production environment. Typically, fixtures will define data like “Test car 1″ and “Test car 2″. This is very useful to during development, but if you have a lot of test data you can crowd your production database pretty quickly. Since there is no mechanism to conditionally load fixture data based on the environment, fixtures lose their appeal. If you must load fixture data into your production database, you may use

$ rake RAILS_ENV=production db:fixtures:load

Second, create the Car model and associate it with a CarType:

$ ruby script/generate model Car

Car < ActiveRecord::Base
  belongs_to :car_type
end

Your Car migration must hold the appropriate car_type_id:

class CreateCars < ActiveRecord::Migration
  def self.up
    create_table :cars do |t|
      t.column :name, :string
      t.column :car_type_id, :int
    end
  end
 
  def self.down
    drop_table :cars
  end
end

Then, you can write code like this:

c = Car.find(1)
c.car_type
=> #"sedan", "id"=>"1"}>

I like to simply even more, so I generally add a method like this to the Car model:

def type
  car_type.name
end

Resulting in the following:

c.type
=> "sedan"

And thats it!

You might be interested to know that you can add a corresponding has_many to the CarType model to easily find all Cars of a particular type:

CarType < ActiveRecord::Base
  has_many :cars
end

And then call something like:

CarType.find(1).cars
=> [#"Test car 1", "id"=>"1", "car_type_id"=>"1"}>]

And I’m not referring to the Rails 2.0 preview. This is Rails 1.2.5, which contains fixes for a JSON XSS (cross-site scripting) vulnerability. I’m not horribly familiar with the details, but the site does say that you don’t have to worry about it if you’re not using JSON. Probably a good idea to upgrade anyway.

>> 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.

I followed them over and over again but couldn’t get my code to work. I knew I had the basic structure setup correctly, since the examples are pretty straightforward, and the concept is not difficult. My has_and_belongs_to_many code originally looked like this:

class Soda < ActiveRecord::Base
  has_and_belongs_to_many :distributors
end
 
class Distributor < ActiveRecord::Base
  has_and_belongs_to_many :sodas
end

Of course there was also a many-to-many join table migration:

class DistributorsSodasJoinTable < ActiveRecord::Migration
  def self.up
    create_table :distributors_sodas, :id => false do |t|
      t.column :soda_id, :int
      t.column :distributor_id, :int
    end
  end  
 
  def self.down
    drop_table :distributors_sodas
  end
end

This works quite nicely:

>> Soda.find(1).distributors
=> []

Later found that I needed to add attributes in the join table to associate extra fields on the Distributors <-> Sodas relationship. has_and_belongs_to_many does not have a Rails way to access those extra fields in the join table. I’ve successfully done it through SQL, but much guilt and remorse lead me to finally learn has_many :through.

This was my best initial attempt:

class Soda < ActiveRecord::Base
  has_many :distributors_sodas
  has_many :distributors, :through =>; :distributors_sodas
end
 
class Distributor < ActiveRecord::Base
  has_many :distributors_sodas
  has_many :distributors, :through => :distributors_sodas
end
 
class DistributorsSodas < ActiveRecord::Base
  belongs_to :soda
  belongs_to :distributor
end

All goes well until I try to do a quick test:

>> Soda.find(1).distributors
NameError: uninitialized constant Soda::DistributorsSoda
...
        from (irb):4

Umm… what? I never tried to instantiate an object of the type Soda::DistributorsSoda. Instead, I was simply trying to use the DistributorsSodas ActiveRecord object, right?

It turns out that has_many :through (apparently) can’t handle using the join tables created by has_and_belongs_to_many. Its just a naming issue – has_many :through will work fine using a one-to-many join table like distributor_sodas (note the missing ’s’ on distributor). If you need a many-to-many join, you have to rename the table to fix the (pluralization?) problem. I deleted the DistributorsSodas model and created the Store model.

class Soda < ActiveRecord::Base
  has_many :stores
  has_many :distributors, :through => :stores
end
 
class Distributor < ActiveRecord::Base
  has_many :stores
  has_many :sodas, :through => :stores
end
 
class Store < ActiveRecord::Base
  belongs_to :soda
  belongs_to :distributor
end

This proved a much better result:

>> Soda.find(1).distributors
=> []

In the end, the association naming convention actually make more sense. I was bummed to have to change the table/model names though.

Please comment if you know how to create the association without changing the model name.