how can I set attributes in the intermediate table for a many-to-many situation??? (e.g. when trying to use the << method)

It seems for the model scenario I have below if there was NO “amount” field on AiAllocation I could go:
.business_items <<
QUESTION: What’s the easiest way to allocate an account item to a business item in this case. I really want something like:
.business_items << ,
Model Scenario (see below)
class AccountItem < ActiveRecord::Base
has_many :ai_allocations
has_many :business_items, :through => :ai_allocations

class AiAllocation < ActiveRecord::Base
# Amount field
belongs_to :account_item
belongs_to :business_item

class BusinessItem < ActiveRecord::Base
has_many :ai_allocations
has_many :account_items, :through => :ai_allocations