如何将 Rails 数据类型更改为 PostGres bigint？

Question

我正在尝试运行以下 Rails 迁移，使用 Rails 4.2.4 和 PostGresql …

class ChangeTimeInMsColInMyObjectTimes < ActiveRecord::Migration
  def change
    change_column :my_object_times, :time_in_ms, :integer, :limit => 8
  end
end

但它会导致以下错误。我

rake db:migrate
== 20160613195631 ChangeTimeInMsColInMyObjectTimes: migrating =====================
-- change_column(:my_object_times, :time_in_ms, :integer, {:limit=>8})
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 bigint
HINT:  You might need to specify "USING time_in_ms::bigint".
: ALTER TABLE "my_object_times" ALTER COLUMN "time_in_ms" TYPE bigint
/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'

我需要在迁移中使用的正确语法是什么？

谢谢，-戴夫

Answer 1

如错误消息所述，您无法自动将 interval 转换为 bigint。你也不能简单地转换它，所以建议的 using time_in_ms::bigint 也不起作用。

虽然这个建议部分正确，但在更改列的类型时手动整理类型转换的常用方法是在 ALTER TABLE 中使用 USING 子句，因此您需要做的就是找出哪个用于将 interval 转换为毫秒数的表达式 bigint.

在数据库中对时间类型进行切片和切块的常用方法是使用 extract:

9.9.1. EXTRACT, date_part
EXTRACT(field FROM source)
The extract function retrieves subfields such as year or hour from date/time values. [...] The extract function returns values of type double precision.

在这种情况下，字段是您感兴趣的时间部分，epoch 可能是您所追求的：

epoch
[...] for interval values, the total number of seconds in the interval

extract 以双精度形式给出秒数，因此您必须自己将秒数转换为毫秒数。类似于：

using extract(epoch from time_in_ms) * 1000

应该可以解决问题。在迁移中，这将是：

change_column :my_object_times, :time_in_ms, :integer, :limit => 8, :using => 'extract(epoch from time_in_ms) * 1000'

或

change_column :my_object_times, :time_in_ms, :bigint, :using => 'extract(epoch from time_in_ms) * 1000'

如何将 Rails 数据类型更改为 PostGres bigint？

How do I change my Rails datatype to PostGres bigint?

postgresql

ruby-on-rails-4

migration

rails-activerecord

bigint