PostgreSQL不能將類型JSON轉換爲字符變化[]

在我的Rails 5.0.5應用程序中，我需要將json列轉換爲string, array: true。PostgreSQL不能將類型JSON轉換爲字符變化[]

我json列中的值是這樣的：

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

我試過這種遷移：

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

，並得到這個錯誤：

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[]

然後我編輯的遷移：

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

終於拿到了這個錯誤：

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

我怎樣才能做到這一點的遷移？

來源

2017-09-30 Georgy Yuriev

我猜數組元素是文件名。如果是這樣，那麼你就可以刪除所有的字符[]"和空格，結果拆到陣列，像這樣：

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)

所以要用：

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

來源

2017-09-30 14:55:00 klin

PostgreSQL不能將類型JSON轉換爲字符變化[]

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