Ruby on rails PostgreSQL可以'；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

my

json

列中的值如下所示：

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

我尝试了这种迁移：

class ChangeTaskAttachmentsTypeToString

得到了这个错误：

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[]

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

然后我编辑了迁移：

class ChangeTaskAttachmentsTypeToString

最后得到了这个错误：

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[]

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

---

如何进行此迁移？

我猜数组元素是文件名。如果是这样，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)

因此，请使用：

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