migration.rb

class Migration

  class << self
    
    attr_accessor :model
    
    def set_model(model)
      @model = eval(model)
    end
    
  end

  def run
    self.class.model.count
    records = self.class.model.find
  end
  
end

migration_spec.rb

require 'spec_helper.rb'

describe "Migration" do
  
  before do
    @record = mock('record', {:name => 'Pete'})
    MyModel = mock('MyModel Class', {:count => 1, :find => [@record]})
    require 'my_migration'
    @migration = MyMigration.new
  end
  
  it "should get a count of the records" do
    MyModel.should_receive(:count)
    @migration.run
  end
  
  it "should find the records" do
    MyModel.should_receive(:find)
    @migration.run
  end
  
  after do
    Object.send(:remove_const, :MyModel)
  end
  
end

my_migration.rb

require 'migration'

class MyMigration < Migration
  
  set_model 'MyModel'
  
end

spec_helper.rb

begin
  require 'spec'
rescue LoadError
  require 'rubygems'
  gem 'rspec'
  require 'spec'
end

# Spec::Runner.configure do |config|
#   config.mock_with :flexmock
# end