Open connection to mysql DB in the rake task
ActiveRecord::Base.establish_connection(
    :adapter  => "mysql2",
    :database => "foo",
    :user => "root",
    :password => "",
)

class MyCustomClass < ActiveRecord::Base
  establish_connection(
   :adapter  => "mysql2",
   :database => "foo",
   :user => "root",
   :password => "",
  )
end

MyCustomClass.find(1)

client = Mysql2::Client.new(:host => "localhost", :username => "root", :database => "foobar", :password => "")
users = client.query("SELECT * FROM users")