In this chapter, I’ll discuss the adapter initialization; how Rails 2.x reads from database.yml and launches the database initialization.

Rails Boot

Here is a condensation of the Rails boot process from Rails Guts.  As Mike Gunderloy suggests, it will be really helpful to have a blank rails app open with

 rake gems:freeze 

invoked :

1. script/server (vendor/rails/railties/lib/commands/server.rb) requires environment.rb from the local path (./)
2. in environment.rb

    Rails::Initializer.run do |config| 

3. Initializer.run has a default parameter

    configuration = Configuration.new 

   (that’s Rails::Configuration in railties/lib/initializer.rb)

4. Configuration#initialize is long, including

    self.database_configuration_file = default_database_configuration_file 

5. Configuration#default_database_configuration_file is :

   def default_database_configuration_file
    File.join(root_path, 'config', 'database.yml')
    end
   

6. Notice that the default command on Rails::Initializer#run was :process, so Initializer#process is now called, which includes #initialize_database …

   def initialize_database
    if configuration.frameworks.include?(:active_record)
    ActiveRecord::Base.configurations = configuration.database_configuration
    ActiveRecord::Base.establish_connection
    end
    end
   

   1. Configuration#database_configuration is where the database.yml is loaded and returned to ActiveRecord::Base.configurations :

      def database_configuration
       require 'erb'
       YAML::load(ERB.new(IO.read(database_configuration_file)).result)
       end
      

7. Now were getting somewhere!!  AR::Base#establish_connection(nil) :
   1. because the call is with spec = nil, #establish_connection is re-executed with RAILS_ENV
   2. this time through, spec = RAILS_ENV (ie, :development), so #establish_connection is called again, this time with the database.yml hash-key of RAILS_ENV (from Configuration#database_configuration)
   3. 3rd time’s a charm!  Now, with spec set to config hash, require the adapter gem,

       gem "activerecord-#{spec[:adapter]}-adapter" 

      and execute the adapter-method (which is expected to be def’d from the adapter’s gem) :

       adapter_method = "#{spec[:adapter]}_connection" 

The adapter-method is the entry-point of our new adapter, so in my next post I’ll pick-apart the #establish_connection process and prepare for our multi-connection pooling adapter’s version.