Rails 5 db migration：如何修复ActiveRecord :: ConcurrentMigrationError
之前的rake db：rollback停滞不前。 现在，在尝试新迁移时，我们收到以下错误：
rake aborted! ActiveRecord::ConcurrentMigrationError: Cannot run migrations because another migration process is currently running. /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord- 5.1.4/lib/active_record/migration.rb:1315:in `with_advisory_lock' /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-5.1.4/lib/active_record/migration.rb:1148:in `migrate' /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-5.1.4/lib/active_record/migration.rb:1007:in `up' /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-5.1.4/lib/active_record/migration.rb:985:in `migrate' /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-5.1.4/lib/active_record/tasks/database_tasks.rb:171:in `migrate' /home/me/.rvm/gems/ruby-2.4.1@global/gems/activerecord-5.1.4/lib/active_record/railties/databases.rake:58:in `block (2 levels) in ' /home/me/.rvm/gems/ruby-2.4.1/gems/rake-12.1.0/exe/rake:27:in `' /home/me/.rvm/gems/ruby-2.4.1/bin/ruby_executable_hooks:15:in `eval' /home/me/.rvm/gems/ruby-2.4.1/bin/ruby_executable_hooks:15:in `' Tasks: TOP => db:migrate (See full trace by running task with --trace) 我们正在使用Postresql
在Rails 5中添加了建议锁定，以防止在迁移期间出现意外的并发错误。 修复方法是清除留在原位的DB锁。
通过针对您的数据库运行此SQL来检查锁：
SELECT DISTINCT age(now(), query_start) AS age, pg_stat_activity.pid,pg_locks.granted,pg_stat_activity.application_name,pg_stat_activity.backend_start, pg_stat_activity.xact_start, pg_stat_activity.state_change, pg_stat_activity.waiting, pg_stat_activity.state, pg_stat_activity.query_start, left(pg_stat_activity.query, 60) FROM pg_stat_activity, pg_locks WHERE pg_locks.pid = pg_stat_activity.pid
要清除锁定，请针对您的数据库运行此SQL：
select pg_advisory_unlock({the pid of the lock you want to release})
如果您正在使用数据库中的数据进行操作，而不是使用表格和列设置，那么一次rake任务可能是更好的地方吗？