issue.rb

class Issue < ActiveRecord::Base
  # This is a very simplifed version of the old method.
  def merge_issues(sources, options = {})
    target = self
    comment = options[:comment]

    transaction do
      sources.each do |source|
        source.merge_into(target, :comment => comment)
      end
    end
  end
end

issue_merger.rb

class IssueMerger
  # This is where I'd like to go.
  def self.merge(options = {})
    target  = options[:target]
    sources = options[:sources]
    comment = options[:comment]

    # After my original tweet I switched to using a dependency container; I
    # set a class variable to ActiveRecord::Base in a Rails initializer. If
    # not set, it's a dummy that simply calls the block. This feels slightly
    # overengineered, but allows me to stub everything out in my tests.
    transaction do
      sources.each do |source|
        source.merge_into(target, :comment => comment)
      end
    end
  end
end