我看过Rails指南和Rails API，但我不明白可逆和恢复的用法。
例如，请参阅此处链接的示例http://guides.rubyonrails.org/migrations.html#using-reversible，并包含在下面：
上面写着Complex migrations may require processing that Active Record doesn't know how to reverse. You can use reversible to specify what to do when running a migration what else to do when reverting it. For example,

class ExampleMigration < ActiveRecord::Migration
  def change
    create_table :products do |t|
      t.references :category
    end

reversible do |dir|
  dir.up do
    #add a foreign key
    execute <<-SQL
      ALTER TABLE products
        ADD CONSTRAINT fk_products_categories
        FOREIGN KEY (category_id)
        REFERENCES categories(id)
    SQL
  end
  dir.down do
    execute <<-SQL
      ALTER TABLE products
        DROP FOREIGN KEY fk_products_categories
    SQL
  end
end

add_column :users, :home_page_url, :string
rename_column :users, :email, :email_address
end

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我知道down部分的代码将在回滚时运行，但为什么要在up块中包含代码？我还看到另一个例子，它有一个可逆的部分，只有一个up块。这样的代码的目的是什么？
最后，我不理解revert。下面是Rails指南中包含的示例，但对我来说意义不大。

`The revert method also accepts a block of instructions to reverse. This could be useful to revert selected parts of previous migrations. For example, let's imagine that ExampleMigration is committed and it is later decided it would be best to serialize the product list instead. One could write:
class SerializeProductListMigration < ActiveRecord::Migration
  def change
    add_column :categories, :product_list

reversible do |dir|
      dir.up do
        # transfer data from Products to Category#product_list
      end
      dir.down do
        # create Products from Category#product_list
      end
    end

    revert do
      # copy-pasted code from ExampleMigration
      create_table :products do |t|
        t.references :category
      end

      reversible do |dir|
        dir.up do
          #add a foreign key
          execute <<-SQL
            ALTER TABLE products
              ADD CONSTRAINT fk_products_categories
              FOREIGN KEY (category_id)
              REFERENCES categories(id)
          SQL
        end
        dir.down do
          execute <<-SQL
            ALTER TABLE products
              DROP FOREIGN KEY fk_products_categories
          SQL
        end
      end

      # The rest of the migration was ok
    end
  end
end`

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