Rails和Postgresql迁移更改列类型

Question

我需要在rails中更改列类型从时间到日期时间。我无法更改列类型。它显示此错误

"rake aborted!
 StandardError: An error has occurred, this and all later migrations canceled:
PG::DatatypeMismatch: ERROR:  column "fun_from" cannot be cast automatically to type timestamp without time zone
HINT:  Specify a USING expression to perform the conversion.
 : ALTER TABLE "general_infos" ALTER COLUMN "fun_from" TYPE timestamp/home/PCA/db/migrate/20150730102811_change_data_type_in_general_infos.rb:3:in `change'
 ActiveRecord::StatementInvalid: PG::DatatypeMismatch: ERROR:  column "fun_from" cannot be cast automatically to type timestamp without time zone
 HINT:  Specify a USING expression to perform the conversion.
:     ALTER TABLE "general_infos" ALTER COLUMN "fun_from" TYPE timestamp
/home/PCA/db/migrate/20150730102811_change_data_type_in_general_infos.rb:3:in `change'
PG::DatatypeMismatch: ERROR:  column "fun_from" cannot be cast automatically to type timestamp without time zone
HINT:  Specify a USING expression to perform the conversion.
/home/PCA/db/migrate/20150730102811_change_data_type_in_general_infos.rb:3:in `change'"

我试过以下格式，

change_column(:table_name, :column_name, :datetime )

并尝试以下格式

change_column :table_name, :column_name, 'datetime USING CAST(column_name AS datetime)'

psql版本 - 9.4.4＆amp;任何人都给我解决方案。