#ActiveRecord, count may not equal to length

child.rb

class Child < ActiveRecord::Base
  belongs_to :parent
end

parent.rb

class Parent < ActiveRecord::Base
  has_many :child, :dependent => :destroy
  accepts_nested_attributes_for :child, :allow_destroy => true
end

script.rb

parent = Parent.new
parent.child.build

parent.child
# => #<ActiveRecord::Associations::CollectionProxy [#<Child id: nil, parent_id: nil, created_at: nil, updated_at: nil>]>

parent.child.length
# => 1
parent.child.count
# => 0
parent.child.size
# => 1