Establishes the connection to the database. Accepts a hash as input where the :adapter key must be specified with the name of a database adapter (in lower-case) example for regular databases (MySQL, Postgresql, etc):

ActiveRecord::Base.establish_connection(
  :adapter  => "mysql",
  :host     => "localhost",
  :username => "myuser",
  :password => "mypass",
  :database => "somedatabase"
)

Example for SQLite database:

ActiveRecord::Base.establish_connection(
  :adapter => "sqlite",
  :database  => "path/to/dbfile"
)

Also accepts keys as strings (for parsing from yaml for example):

ActiveRecord::Base.establish_connection(
  "adapter" => "sqlite",
  "database"  => "path/to/dbfile"
)

The exceptions AdapterNotSpecified, AdapterNotFound and ArgumentError may be returned on an error.

Source Code

# File active_record/connection_adapters/abstract/connection_specification.rb, line 204
def self.establish_connection(spec = nil)
  case spec
    when nil
      raise AdapterNotSpecified unless defined? RAILS_ENV
      establish_connection(RAILS_ENV)
    when ConnectionSpecification
      clear_active_connection_name
      @active_connection_name = name
      @@defined_connections[name] = spec
    when Symbol, String
      if configuration = configurations[spec.to_s]
        establish_connection(configuration)
      else
        raise AdapterNotSpecified, "#{spec} database is not configured"
      end
    else
      spec = spec.symbolize_keys
      unless spec.key?(:adapter) then raise AdapterNotSpecified, "database configuration does not specify adapter" end

      begin
        require 'rubygems'
        gem "activerecord-#{spec[:adapter]}-adapter"
        require "active_record/connection_adapters/#{spec[:adapter]}_adapter"
      rescue LoadError
        begin
          require "active_record/connection_adapters/#{spec[:adapter]}_adapter"
        rescue LoadError
          raise "Please install the #{spec[:adapter]} adapter: `gem install activerecord-#{spec[:adapter]}-adapter` (#{$!})"
        end
      end

      adapter_method = "#{spec[:adapter]}_connection"
      if !respond_to?(adapter_method)
        raise AdapterNotFound, "database configuration specifies nonexistent #{spec[:adapter]} adapter"
      end

      remove_connection
      establish_connection(ConnectionSpecification.new(spec, adapter_method))
  end
end