在架构中我有这个：
add_index "key_performance_indicators", ["organization_id"], :name => "index_key_performance_inds_on_organization_id"
我想编写一个 remove_index 迁移来删除这个索引，但不知道我应该使用什么索引名称？是key_performance_inds还是performance_inds_on_organization_id？或者是其他东西？
如果我使用可视化管理工具，它会显示：
您将:name选项传递给，add_index因此索引的名称就是您设置的名称。在这种情况下，index_key_performance_inds_on_organization_id因此将其删除：
remove_index 'key_performance_indicators', :name => 'index_key_performance_inds_on_organization_id'
Tomdarkness has provided the right answer but you might want to omit the :name part altogether unless there is a need to conform to a specific database schema. add_index will choose a sensible index name by default for you and remove_index will use that same default.
For example:
add_index :key_performance_indicators, :organization_id
The above line will add the index with the name index_key_performance_indicators_on_organization_id and the following will remove it:
remove_index :key_performance_indicators, :organization_id