Newbie implementing one to many many to one relationship

I am attempting to tie a Project to Projectparts to Parts and I
believe the relationship should be one-to-many, then many-to-one. My
goal is to build the Parts table so that this table can grow and the
Parts descriptions can be re-used in different Projects (ultimately
tie this in to an autocomplete plug-in so the user doesn’t have to re-
type a part every time, but I’m not there yet). I had the application
working through to the Project - Projectparts relationship, I am now
trying to add the description component from the Part table.

I think I’ve tracked down my problem to the _projectpart partial of
code that I’ve implemented for this. Here’s the partial:

<div class="fields">
  <%= f.text_field :qty, :size => 3 %>

  <% f.fields_for :parts do |builder| %>      #  <======I think

problem is here.
<%= builder.text_field :description, :size => 50 %>
<% end %>

  <%= remove_child_link "Delete", f %>
</div>

As the code is now, the ‘New’ view will render, but when I save the
data from the UI, I get an unknown attribute error in my ‘Create’:

ActiveRecord::UnknownAttributeError in ProjectsController#create

unknown attribute: parts

However, when I change the :parts reference to :part, the New screen
will not render & I get a nil object error:

NoMethodError in Projects#new

Showing app/views/projects/_projectpart.html.erb where line #10

raised:

You have a nil object when you didn't expect it!
You might have expected an instance of ActiveRecord::Base.
The error occurred while evaluating nil.new_record?

So I’m thinking I’m not referencing this correctly. I’ve tried using
@project.projectpart.description, @project.projectpart & other
variations, but not sure if the model is set up correctly in the first
place (the description is the ‘one’ at the end of the one-to-many /
many-to-one relationship).

Any thoughts? Thanks in advance.

For reference:

Models:

class Project < ActiveRecord::Base
validates_presence_of :name
has_many :projectparts, :dependent => :destroy
accepts_nested_attributes_for :projectparts, :reject_if => lambda
{ |a| a.values.all?(&:blank?) }, :allow_destroy => true

def projectpart_attributes=(projectpart_attributes)
  projectpart_attributes.each do |attributes|
  projectpart.build(attributes)
  end
end

end

class Projectpart < ActiveRecord::Base
belongs_to :project
has_one :part
accepts_nested_attributes_for :parts, :reject_if => lambda { |a|
a.values.all?(&:blank?) }
end

class Part < ActiveRecord::Base
has_and_belongs_to_many :projectparts
end

My db schema - the foreign key for accessing the Part description is
in the projectparts table since there should be many of these records
and I don’t want to unnecessarily increase the number of records in
the Parts table, but perhaps this is causing part of the problem?

create_table “projects”, :force => true do |t|
t.string “name”
t.string “description”
end

create_table “projectparts”, :force => true do |t|
t.integer “project_id”
t.integer “part_id”
t.datetime “created_at”
t.datetime “updated_at”
t.integer “qty”
end

create_table “parts”, :force => true do |t|
t.string “description”
t.datetime “created_at”
t.datetime “updated_at”
end

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