3.0.10

$ ruby ~/Desktop/test.rb 
Active Record 3.0.10
-- create_table(:foos)
   -> 0.6120s
-- create_table(:bars)
   -> 0.0008s
size: 0
count: 0
length: 0

3.1.0

$ ruby ~/Desktop/test.rb 
Active Record 3.1.0
-- create_table(:foos)
   -> 0.0356s
-- create_table(:bars)
   -> 0.0008s
size: 1
count: 0
length: 1

gistfile1.rb

gem 'rails', '3.0.0'
# gem 'rails', '3.1.0'

require 'active_record'

puts "Active Record #{ActiveRecord::VERSION::STRING}"

ActiveRecord::Base.establish_connection(
  :adapter  => 'sqlite3',
  :database => ':memory:'
)

ActiveRecord::Schema.define do
  create_table :foos
  create_table :bars do |t|
    t.integer :foo_id
  end
end

class Foo < ActiveRecord::Base
  has_many :bars
end

class Bar < ActiveRecord::Base
end

foo = Foo.create!
foo.bars.new

puts "size: #{foo.bars.size}"
puts "count: #{foo.bars.count}"
puts "length: #{foo.bars.length}"