Question

The error shows this code can not rollback:

class AddCountToTag < ActiveRecord::Migration
    def change
        change_table :tags do |t|
            t.integer :count
            t.index :count
        end
    end
end

Where is wrong?

Answer 1

change_table is not supported yet for reversible migrations. See this comment at the top of the file (quoted below) :

# <tt>ActiveRecord::Migration::CommandRecorder</tt> records commands done during
    # a migration and knows how to reverse those commands. The CommandRecorder
    # knows how to invert the following commands:
    #
    # * add_column
    # * add_index
    # * add_timestamps
    # * create_table
    # * create_join_table
    # * remove_timestamps
    # * rename_column
    # * rename_index
    # * rename_table

If you need to be able to reverse, you can use add_column instead of change_table:

class AddCountToTag < ActiveRecord::Migration
    def change
        add_column :tags, :count, :integer
        ...
    end
end