Capistrano with PostgreSQL, error: database is being accessed by other users - ruby-on-rails

Question

I have a Rails app that uses PostgreSQL as a backend with a cert environment that tries to mimic production, except that it needs to have the database reset periodically for QA.
When I attempt to execute db:reset from a Capistrano task during deployment I get the error:
ERROR: database "database_name" is being accessed by other users
and the database cannot be dropped as part of the reset task resulting in deployment failing. Is there a way I can reset database connections from Capistrano so I can successfully drop the table? Piping the SQL to psql from a Capistrano task might work but I was wondering if there was a better way to go about this.

Answer 1

With PostgreSQL you can issue the following statement to return the backend pids of all open connections other than then this one:
SELECT pid FROM pg_stat_activity where pid <> pg_backend_pid();
Then you can issue a a termination request to each of those backends with
SELECT pg_terminate_backend($1);
Binding the pids returned from the first statement to each pg_terminate_backend exec.
If the other connections are not using the same user as you, you will have to connect as a superuser to successfully issue the terminates.
Admin signalling functions docs
Monitoring stats functions
pg_stat_activity view docs
UPDATE: Incorporating comments and expressing as Capistrano task:
desc "Force disconnect of open backends and drop database"
task :force_close_and_drop_db do
dbname = 'your_database_name'
run "psql -U postgres",
:data => <<-"PSQL"
REVOKE CONNECT ON DATABASE #{dbname} FROM public;
ALTER DATABASE #{dbname} CONNECTION LIMIT 0;
SELECT pg_terminate_backend(pid)
FROM pg_stat_activity
WHERE pid <> pg_backend_pid()
AND datname='#{dbname}';
DROP DATABASE #{dbname};
PSQL
end

Answer 2

I have combined dbenhur's answer with this Capistrano task to achieve the result I needed works like a charm:
desc 'kill pgsql users so database can be dropped'
task :kill_postgres_connections do
run 'echo "SELECT pg_terminate_backend(procpid) FROM pg_stat_activity WHERE datname=\'database_name\';" | psql -U postgres'
end
This assumes the auth_method for user postgres set to 'trust' in pg_hba.conf
Then you can just call it in your deploy task after update_code and before migrate
after 'deploy:update_code', 'kill_postgres_connections'

Answer 3

You can simply monkeypatch the ActiveRecord code that does the dropping.
For Rails 3.x:
# lib/tasks/databases.rake
def drop_database(config)
raise 'Only for Postgres...' unless config['adapter'] == 'postgresql'
Rake::Task['environment'].invoke
ActiveRecord::Base.connection.select_all "select pg_terminate_backend(pg_stat_activity.pid) from pg_stat_activity where datname='#{config['database']}' AND state='idle';"
ActiveRecord::Base.establish_connection config.merge('database' => 'postgres', 'schema_search_path' => 'public')
ActiveRecord::Base.connection.drop_database config['database']
end
For Rails 4.x:
# config/initializers/postgresql_database_tasks.rb
module ActiveRecord
module Tasks
class PostgreSQLDatabaseTasks
def drop
establish_master_connection
connection.select_all "select pg_terminate_backend(pg_stat_activity.pid) from pg_stat_activity where datname='#{configuration['database']}' AND state='idle';"
connection.drop_database configuration['database']
end
end
end
end
(from: http://www.krautcomputing.com/blog/2014/01/10/how-to-drop-your-postgres-database-with-rails-4/)