ruby-on-rails - PostgreSQL无法将json类型转换为字符变化[]

在我的Rails 5.0.5应用程序中，我需要将json列转换为string, array: true。
myjson列中的值如下：

[ "200px-RR5219-0015R.png", "2017_03_25_2235.doc", "137555.jpg" ]

我试过这个迁移：

class ChangeTaskAttachmentsTypeToString < ActiveRecord::Migration[5.0]
  def change
    change_column :tasks, :attachments, :string, array: true
  end
end

得到这个错误：

ActiveRecord::StatementInvalid: PG::DatatypeMismatch: ERROR:  column "attachments" cannot be cast automatically to type character varying[]
HINT:  You might need to specify "USING attachments::character varying[]".
: ALTER TABLE "tasks" ALTER COLUMN "attachments" TYPE character varying[]

然后我编辑了迁移：

class ChangeTaskAttachmentsTypeToString < ActiveRecord::Migration[5.0]
  def change
    change_column :tasks, :attachments, 'character varying[] USING attachments::character varying[]'
  end
end

最后得到这个错误：

PG::CannotCoerce: ERROR:  cannot cast type json to character varying[]
: ALTER TABLE "tasks" ALTER COLUMN "attachments" TYPE character varying[] USING attachments::character varying[]

我怎样才能做这个迁移？

最佳答案

我猜数组元素是文件名。如果是，则可以删除所有字符[]"和空格，并将结果拆分为数组，如下所示：

with my_table(attachments) as (
values
    ('[ "200px-RR5219-0015R.png", "2017_03_25_2235.doc", "137555.jpg" ]'::json)
)
select string_to_array(translate(attachments::text, '[] "', ''), ',')::varchar[]
from my_table;

                     string_to_array
---------------------------------------------------------
 {200px-RR5219-0015R.png,2017_03_25_2235.doc,137555.jpg}
(1 row)

所以使用：

... USING string_to_array(translate(attachments::text, '[] "', ''), ',')::varchar[]