如何在 Rails 迁移中将列类型更改为间隔？

Question

我正在使用 Rails 4.2.3 和 PostGre 数据库。我正在尝试将 table 中的列类型更改为“间隔”类型，因此我尝试了此迁移

class ChangeTimeInMsInMyObjectTimes < ActiveRecord::Migration
  def change
    change_column :my_object_times, :time_in_ms, :interval
  end
end

但是在运行“rake db:migrate”时，我得到了下面令人失望的错误……

== 20160530164019 ChangeTimeInMsInMyObjectTimes: migrating ========================
-- change_column(:my_object_times, :time_in_ms, :interval)
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::DatatypeMismatch: ERROR:  column "time_in_ms" cannot be cast automatically to type interval
HINT:  You might need to specify "USING time_in_ms::interval".
: ALTER TABLE "my_object_times" ALTER COLUMN "time_in_ms" TYPE interval
/Users/davea/.rvm/gems/ruby-2.3.0@global/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/postgresql/database_statements.rb:155:in `async_exec'
/Users/davea/.rvm/gems/ruby-2.3.0@global/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/postgresql/database_statements.rb:155:in `block in execute'
/Users/davea/.rvm/gems/ruby-2.3.0@global/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/abstract_adapter.rb:472:in `block in log'
/Users/davea/.rvm/gems/ruby-2.3.0@global/gems/activesupport-4.2.5.1/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
/Users/davea/.rvm/gems/ruby-2.3.0@global/gems/activerecord-4.2.5.1/lib/active_record/connection_adapters/abstract_adapter.rb:466:in `log'

如何更改我的迁移以使其正常工作？

Answer 1

问题是 Postgres 不知道如何将整数（我假设 time_in_ms 现在就是这样。）转换为一个区间。但是您可以使用 USING 关键字告诉它如何操作。它采用 Postgres 将用来转换所有旧值的表达式。因此，您可以在迁移中使用它：

change_column :my_object_times, :time_in_ms,
              "interval USING (time_in_ms || ' milliseconds')::interval"

请注意，如果您这样做，您应该编写单独的 up 和 down 方法，因为这种使用 change_column 的方式不会自动逆转。

哦还有：你可以忽略@Boltz0r 的要点。正如@mu 所说，它正试图解决一个不同的问题。

Answer 2

尝试

class ChangeTimeInMsInMyObjectTimes < ActiveRecord::Migration
  def change
    remove_column :my_object_times, :time_in_ms, :your_current_type
    add_column :my_object_times, :time_in_ms, :interval
  end
end

如何在 Rails 迁移中将列类型更改为间隔？

How do I change my column type to interval in my Rails migration?

migration

postgresql

intervals

column-types

ruby-on-rails-4