app-models-pet.rb

class Pet < ActiveRecord::Base
  has_many :user_pets
  belongs_to :user, :through => :user_pets
  validates :name, :allow_nil => true, :inclusion => { :in => Pet.valid_pets }


  def self.valid_pets
    ['fish',
     'bird',
     'tree',
    ]
  end
end

app-models-user.rb

class User < ActiveRecord::Base
  has_many :user_pets
  has_many :pets, :through => :user_pets
end

app-models-user_pets.rb

class UserPet  < ActiveRecord::Base
  belongs_to :user
  belongs_to :pet
end

class CreateUserPets < whatever::migration

create_table :user_pets do |t|
  t.integer :user_id
  t.integer :pet_id
end
  

app-views-whatever-show.html.erb

<%# I don't remember select_for_options or select_for_options_from_hash syntax, so I'll just display the list of valid pets here for you: %>
<% Pet.valid_pets.join(',') %>

model.rb

class Pet
  belongs_to :user, :through => :users_pets
  validates :name, :allow_nil => true, :inclusion => { :in => Pet.valid_pets }


  def self.valid_pets
    ['fish',
     'bird',
     'tree',
    ]
  end
end

view.html.erb

<%# I don't remember select_for_options or select_for_options_from_hash syntax, so I'll just display the list of valid pets here for you: %>
<% Pet.valid_pets.join(',') %>