model.association.delete_all performance issue

association_delete_all_test.rb

require 'active_record'
require 'logger'

# Print out what version we're running
puts "Active Record #{ActiveRecord::VERSION::STRING}"

# Connect to an in-memory sqlite3 database (more on this in a moment)
ActiveRecord::Base.establish_connection(:adapter  => 'sqlite3', :database => ':memory:')

# Create the minimal database schema necessary to reproduce the bug
ActiveRecord::Schema.define do
  create_table :discussions, :force => true do |t|
  end

  create_table :posts, :force => true do |t|
    t.integer :discussion_id
  end
end

# Create the minimal set of models to reproduce the bug
class Discussion < ActiveRecord::Base
  has_many :posts, :dependent => :delete_all
end

class Post < ActiveRecord::Base
  belongs_to :discussion
end

# Create some test data
#
# If you're demonstrating an exception, then this is probably not necessary,
# but if your bug is to do with the wrong data being returned from the database,
# then you'll probably need some test data to show that.
discussion = Discussion.create!
ActiveRecord::Base.logger = Logger.new(nil)
10000.times do
  Post.create(:discussion => discussion)
end

# Reproduce the actual bug!
ActiveRecord::Base.logger = Logger.new(STDOUT)
Discussion.first.posts.delete_all