Ruby on rails 在Heroku中更改列类型

在尝试更改表列类型时，执行heroku run rake db:migrate命令时遇到问题。我在stack overflow上看到您必须对PG使用一个特殊命令。然后我尝试运行rails d migration ChangeExpirationDateType，但它似乎不起作用。我的问题是，我如何删除更改列迁移，或者只是解决这个问题

> heroku run rake db:migrate
Running `rake db:migrate` attached to terminal... up, run.8613
==  ChangeExpirationDateType: migrating =======================================
-- change_column(:users, :expiration_date, :date)
rake aborted!
StandardError: An error has occurred, this and all later migrations canceled:

PG::DatatypeMismatch: ERROR:  column "expiration_date" cannot be cast automatically to type date
HINT:  Specify a USING expression to perform the conversion.
: ALTER TABLE "users" ALTER COLUMN "expiration_date" TYPE date/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:128:in `async_exec'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:128:in `block in execute'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract_adapter.rb:435:in `block in log'
/app/vendor/bundle/ruby/2.0.0/gems/activesupport-4.0.2/lib/active_support/notifications/instrumenter.rb:20:in `instrument'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract_adapter.rb:430:in `log'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:127:in `execute'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/postgresql/schema_statements.rb:389:in `change_column'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:625:in `block in method_missing'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:597:in `block in say_with_time'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:597:in `say_with_time'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:617:in `method_missing'
/app/db/migrate/20140420080240_change_expiration_date_type.rb:3:in `change'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:571:in `exec_migration'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:555:in `block (2 levels) in migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:554:in `block in migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract/connection_pool.rb:294:in `with_connection'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:553:in `migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:709:in `migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:959:in `block in execute_migration_in_transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:1005:in `block in ddl_transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract/database_statements.rb:202:in `block in transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract/database_statements.rb:210:in `within_new_transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/connection_adapters/abstract/database_statements.rb:202:in `transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/transactions.rb:209:in `transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:1005:in `ddl_transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:958:in `execute_migration_in_transaction'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:920:in `block in migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:916:in `each'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:916:in `migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:764:in `up'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/migration.rb:742:in `migrate'
/app/vendor/bundle/ruby/2.0.0/gems/activerecord-4.0.2/lib/active_record/railties/databases.rake:42:in `block (2 levels) in <top (required)>'
Tasks: TOP => db:migrate
(See full trace by running task with --trace)

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现在过期日期是什么类型？一个字符串，我想把它改成日期。字符串的格式是什么？格式是什么意思？数据库中的日期是什么样子的2014-04-20'? '2014年4月20日星期日？还有别的吗？基本上，您需要构建一个将字符串转换为日期的表达式，以便在ALTER表中包含适当的USING子句。