How do I create a form for "has_and_belongs_to_many" association?

Vincent Lin wrote in post #1070656:

git://github.com/vincentopensourcetaiwan/vocabulary.git
This is my source codes.

I got two models, Book and Word.

class Book < ActiveRecord::Base
  has_and_belongs_to_many :words
end

class Word < ActiveRecord::Base
   has_and_belongs_to_many :books
end

I got a Join Tables for has_and_belongs_to_many Associations

class CreateBooksWords < ActiveRecord::Migration
  def self.up
    create_table :books_words, :id => false do |t|
      t.integer :book_id
      t.integer :word_id

      t.timestamps
    end
  end

  def self.down
    drop_table :books_words
  end
end

Now I want to create a form, and user can insert words into a book.
How do I do that?

There are some conceptual issues here.
If this is in fact a many-to-many relationship between books and words
so that, a book can have many words, and a word can be in many books,
then:

Don't think of "i want to add words to a book", but think of "I have a
word, and i have a book, and i want to build a relationship between the
two."
So the form you're talking about is not a form on words and books, but a
form about a single model: book_word_relationship (name it whatever you
like). You will have a relationship_controller which will handle #create
and #destroy actions. At this point the form should be trivial. The
#create action will take as parameters the book_id and word_id. Or more
appropriately @book.book_word_relationships.build(params[:word).

This means you will need a proper model and controller to handle these
relationships. Which also means that a HABTM relationship is not
appropriate and you should use has_many :through=>'relationship'.

This railscast has some good advice about the front-end of a many-many
relationship:
http://railscasts.com/episodes/163-self-referential-association
don't worry about the self-referential part.

It sounds like the question has been sufficiently answered.

But if you are looking for more content on the subject, check out the Rails Guides on Associations.

http://guides.rubyonrails.org/association_basics.html#the-types-of-associations