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% irb 
irb(main):001:0> require 'active_record'
LoadError: no such file to load -- active_record
        from (irb):1:in `require'
        from (irb):1
irb(main):002:0>  require 'rubygems'
=> true
irb(main):003:0> require 'active_record'
=> true
irb(main):004:0> defined? ActiveRecord::Associations::BelongsToPolymorphicAssociation
=> "constant"
irb(main):005:0>