active_record_base_patch.rb

# Override ActiveRecord::Base.new to return an STI subclass if the type field is set.
module ActiveRecord
  class Base
    class << self
      def new(*attributes, &block)
        obj = super
        model = find_sti_class(obj[inheritance_column])
        return obj if obj.is_a? model
        obj.becomes(model)
      end
    end
  end
end

sti_example.rb

# STI models

class Cat < ActiveRecord::Base
end

class FatCat < Cat
end

# Before patch:

Cat.new {|cat| cat.type = 'FatCat' } # => #<Cat id: nil, type: 'FatCat'>
Cat.where(type: 'FatCat').new        # => #<Cat id: nil, type: 'FatCat'>
Cat.where(type: 'FatCat').first      # => #<FatCat id: nil, type: 'FatCat'>

# After patch:

Cat.new {|cat| cat.type = 'FatCat' } # => #<FatCat id: nil, type: 'FatCat'>
Cat.where(type: 'FatCat').new        # => #<FatCat id: nil, type: 'FatCat'>
Cat.where(type: 'FatCat').first      # => #<FatCat id: nil, type: 'FatCat'>