Understanding how establish_connection works in ActiveRecord

Question

This code was taken from ActiveRecord 2.3.14's gem class ConnectionHandler

def establish_connection(name, spec)
  @connection_pools[name] = ConnectionAdapters::ConnectionPool.new(spec)
end

It seems each time ruby calls establish_connection on the model, it's creating a new connection pool.

My question:

If I have 5 models that use establish_connection to the same database, is Rails smart enough to pick an already existing pool rather creating a new one with the same connection credentials? Does this also happen if my 5 models are subclasses of some abstract class that uses establish_connection? Will it always pick a connection from the @connection_pools if it exists?

Update 1

I'm talking about a concrete example. You have 5 models with 5 different connections, each time Rails uses a model it executes establish_connection. Looking at the code in ActiveRecord, when it executes establish_connection it creates a new pool with connections to that specific connection. What I'm wondering is whether each time Rails calls a model's establish_connection, does it create a new pool or take the existing one.

Example: you come to my site and see a product list. You've just hit an action that calls Product.all, which executes establish_connection to some database on Amazon. Then, I come to the product list, what happens? Do I grab the established connection or am I creating a new pool with that connection?

Update 2

My guess is that first time Rails loads my models it's creating pools with different connections. After, when I use some Model.method, it just grabs the connection associated with the model and executes the method.

I'm not sure what happens when 2 models have two equal connections (not in the abstract class but in self class). Will this produce two same connection pools, or is ActiveRecord smart enough to catch this case?

Jeromejeromy

BTW, very good question. – Dardanelles

Answer 1

You really do not have to call establish_connection on each model. You can simply do next:

ActiveRecord::Base.establish_connection(
 { :adapter => 'mysql2',
   :database => 'some_database',
   :host => 'localhost',
   :username => 'root',
   :password => "" }
)

and you will have access to connection. (This chunk of code has been extracted from real code(except database name :) )).
But according to API I think that Rails does not take existing connection from other model (correct me if I am wrong).
Also here is a link to documentation. You can read more about the connection there.
I hope I helped you alittle.

Dardanelles

It grabs, when calling models methods, but in initialize state when it just loads models and executes establish_connection, lookin' at the code it just creates a pool, no matter if it already exists. Keys in @connection_pool hash are different, but values are the same. – Jeromejeromy

If 2 different models will establish connection they will get into pool (establish connection in some models and then type ActiveRecord::Base.connection_handler). I think connections just collecting and each of them can connect to database separately. – Dardanelles

Answer 2

AR calls establish_connection only once, for ActiveRecord::Base. All subclasses use the one connection.

You can manually call establish connection yourself on some subclasses. This is very convenient for using two databases at once, e.g.

class MyMainUser < ActiveRecord::Base; end 
class MyOtherDb < ActiveRecord::Base; end
class MyOtherUser < MyOtherDb; end

MyOtherDb.establish_connection ...

MyMainUser.first # uses default db
MyOtherUser.first # uses other db

You can't do queries that would cross databases though.

Allethrin

I had to add self.abstract_class = true in MyOtherDb class to run inserts. Otherwise it was trying to get the my_other_dbs table not exist. – Bed

Answer 3

This comment:

# Check-out a database connection from the pool, indicating that you want
# to use it. You should call #checkin when you no longer need this.
#
# This is done by either returning an existing connection, or by creating
# a new connection. If the maximum number of connections for this pool has
# already been reached, but the pool is empty (i.e. they're all being used),
# then this method will wait until a thread has checked in a connection.
# The wait time is bounded however: if no connection can be checked out
# within the timeout specified for this pool, then a ConnectionTimeoutError
# exception will be raised.

from: https://github.com/rails/rails/blob/dd944cbf5879e675fff541d1be7c7eb6c3382d01/activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb#L242-251

Should explain the situation

Stagey