Question

I need to share a database handle among a few Ruby scripts. I am using the DBI gem to connect to the database. Consider the following example

#a.rb
class A
  require 'dbi'

  def connect
    if a database handle is already open return it
    else create a new one and return it
  end
end

#b.rb
class B
  require 'a'
  a = A.new
  dbh = a.connect
  #some database queries here
end

#c.rb
class C
  require 'a'
  a = A.new
  dbh = a.connect #Here the database handle created by class B should be returned 
end

I understand that class instance variables are the way to go to achieve the said goal. Can someone please provide some insight on this?

Does DBI have something similar to Log4r

class A
  require 'log4r'
  Log4r::Logger.new('SO') #create a new instance here
end

class B
  require 'a'
  Log4r::Logger['SO'] # get the existing instance here
end

Many thanks.

Answer 1

The following uses a Singleton to manage a hash of DBI handles. It will create a new handle for a given set of connect options (the args one would pass to DBI.connect) the first time a connect is requested with those options. Subsequent requests with the same options will get the already open handle.

# there_can_be_only_one_dbh.rb

require 'dbi'
require 'singleton'

class ThereCanBeOnlyOneDBH
  include Singleton

  DEFAULT_CONNECT_OPTIONS = ['dsn','user','pw']

  def connect( *options )
    options = DEFAULT_CONNECT_OPTIONS unless options and !options.empty?
    @handles ||= {}
    @handles[options.join('|')] ||= DBI.connect( *options )
  end
end

# b.rb
require 'there_can_be_only_one_dbh'

class B
  def initialize( *dbi_connect_options )
    @dbh = ThereCanBeOnlyOneDBH.instance.connect( *dbi_connect_options )
  end

  # methods which issue queries using @dbh
end

# c.rb
require 'there_can_be_only_one_dbh'

class C
  def initialize( *dbi_connect_options )
    @dbh = ThereCanBeOnlyOneDBH.instance.connect( *dbi_connect_options )
  end

  # other methods which issue queries using @dbh
end