[console in rails]. uninitialized constant Class

when i run a method in terminal of rails. first time, it working :

 Spree::Campaign.first
 Campaign Load (0.4ms)  SELECT  `campaigns`.* FROM `campaigns`   ORDER BY `campaigns`.`id` ASC LIMIT 1
 => #<Campaign id: 1, name: "campaign 1", user_id: 1, created_at: "2015-10-27 06:48:01", updated_at: "2015-10-29 04:22:03", description: nil, active: true>

but when i try run code above again

Spree::Campaign.first
NameError: uninitialized constant Spree::Campaign
from (irb):2
from /home/kop/.rvm/gems/ruby-2.1.4@rails3213/gems/railties-4.1.6/lib/rails/commands/console.rb:90:in `start'
from /home/kop/.rvm/gems/ruby-2.1.4@rails3213/gems/railties-4.1.6/lib/rails/commands/console.rb:9:in `start'
from /home/kop/.rvm/gems/ruby-2.1.4@rails3213/gems/railties-4.1.6/lib/rails/commands/commands_tasks.rb:69:in `console'
from /home/kop/.rvm/gems/ruby-2.1.4@rails3213/gems/railties-4.1.6/lib/rails/commands/commands_tasks.rb:40:in `run_command!'
from /home/kop/.rvm/gems/ruby-2.1.4@rails3213/gems/railties-4.1.6/lib/rails/commands.rb:17:in `<top (required)>'
from /home/kop/.rvm/gems/ruby-2.1.4@rails3213/gems/polyglot-0.3.5/lib/polyglot.rb:65:in `require'
from /home/kop/.rvm/gems/ruby-2.1.4@rails3213/gems/polyglot-0.3.5/lib/polyglot.rb:65:in `require'
from /home/kop/rails/donghoxteen/bin/rails:8:in `<top (required)>'
from /home/kop/.rvm/rubies/ruby-2.1.4/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:54:in `require'
from /home/kop/.rvm/rubies/ruby-2.1.4/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:54:in `require'
from -e:1:in `<main>'

Why? and how to fix this error??