migration.rb

class Migration
  class << self
    def create_table(name, &block)
      table = Table.new name
      table.evaluate &block
      table.create
    end

    def drop_table(name)
      table = Table.new name
      table.drop
    end
  end
end

class Table
  def initialize(name)
    @name = name
    @columns = []
  end

  def evaluate(&block)
    instance_eval &block
  end

  def string(*columns)
    @columns << columns
  end

  def create
    #DB logic to create table here
    "Table #{@name} created with columns #{@columns.join(', ')}"
  end

  def drop
    #DB logic to remove table here
    "Table #{@name} dropped from database"
  end
end

class MyMigration < Migration
  def self.up
    create_table 'users' do
      string 'username', 'password'
    end
  end

  def self.down
    drop_table 'users'
  end
end