How does a has_many :through association and its join model attribute interact?

I am having real difficulty getting my head around the issue of setting
this attribute, units_alloc, in my join model?

My models:

  class Contract < AR::Base
    has_many :codelines
    has_many :codes, :through => :codelines

    accepts_nested_attributes_for :codes

    attr_accessible :codes_attributes, :codes;
:authnum,:st_date,:end_date
  end

  class Codeline < AR::Base
    belongs_to :contract
    belongs_to :code
    units_alloc ...... this is the attribute I would like to set
  end

  class Code < AR::Base
    has_many :codelines
    has_many :contracts, :through => :codelines
  end

The new action of my app/controllers/contracts_controller.rb

  def new
    @contract = Contract.new
    @contract.codes.build
  end

the partial for my view in app/views/contracts/_fields.html.haml

    = f.fields_for :codes do |ff|
      .field
         = ff.label :name, "Code Name"
         %br/
         = ff.text_field :code_name
      .field
      .
      .
    = f.fields_for :codelines do |ff|
      .field
        = ff.label :name, "Units Alloc"
        %br/
        = ff.text_field :units_alloc

From reading Rails Guides and watching railscasts #196 and #197 and
researching nested attributes on the internet I came to the
understanding that the @contract.codes.build line in the new action of
my app/controllers/contracts_controller.rb not only built the code
object but that it also built the codelines object. If I leave my new
action of my app/controllers/contracts_controller.rb as above that is
exactly what happens. My codelines table is populated as follows:

    id contract_id code_id units_alloc
     1 1 1 .... @contract.codes.build

But if you look at my view I would actually like to set units_alloc and
this line @contract.codes.build does not give me access to units_alloc
since it is in the codeline model. I added @contract.codelines.build to
the new action of my app/controllers/contracts_controller.rb and now the
units_alloc shows in my view and I am able to set it.
There is a problem though,I now have two rows in my codelines table. One
resulting from the @contract.codes.build where both the code object and
codeline object are built and the second row is as a result of the
@contract.codelines.build. Here is what my codelines table shows:

    id contract_id code_id units_alloc
     1 1 1 ..@contract.codes.build
     2 1 80.00 ..@contract.codelines.build

Should'nt I be able to get access to units_alloc through the first build
given by @contract.codes.build, where both the code object and the
codeline object is built?

Does anyone know if my understanding of this issue is correct or could
you point me to a resource where all will be revealed?

Thanks for any suggestions.