has_many through but with unique source and multiple associations - ruby-on-rails

Question

As a user, I can add my favorite book to my book list.
Another user adds the same book to their book list.
There should only be one instance of the book in the database but 2 user/book associations.
class User
has_many :bookReferences
has_many :books, through: :bookReferences
class Book
validates_uniqueness_of :title
has_many :bookReferences
has_many :users, through: :bookReferences
class BookReference
belongs_to :user
belongs_to :book
The first problem, is that if the book is already in the system, the bookReference is not created for the second user because the book isn't unique.
So, if the book is already in the system, I want to just create the association record in the bookReference table.
The second problem, when a user deletes a book, I only want it to delete the reference to the book unless they are the only user referencing that book.
Basically, this is complete overview of the usage I'm trying to achieve:
user1.books.first
=> id: 1, title: "Moby Dick"
user2.books.first
=> id: 1, title: "Moby Dick"
books.all
=> id: 1, title: "Moby Dick"
user1.books.first.destroy
user1.books.first
=> nil
books.all
=> id: 1, title: "Moby Dick"
user2.books.first
=> id: 1, title: "Moby Dick"
user2.books.first.destroy
=> nil
books.all
=> nil
UPDATE
Based on these answers, perhaps I wasn't very clear. Let me try again...
The book controller, has a basic CRUD create method which:
def create
current_user.books.create(name: params[:book][:title])
With the way the has_many through association is setup currently, the book will only be created if it is unique. If it's already in the system, it will return false. What I WANT it to do is create the association with the existing book as if it was a new record. This is what I am currently doing in my application to accomplish this but it feels wrong:
def create
book = Book.where(name: params[:book][:title]).first_or_create!
BookReference.where(book_id: book.id, user_id: current_user.id).first_or_create!
Then the second problem is when a user removes a book from their account. Doing the traditional CRUD destroy will remove it from all accounts:
def destroy
book = current_user.books.find(params[:id])
book.destroy
So to get around this, I'm currently doing the following. Again though, this doesn't feel "right":
def destroy
book = current_user.books.find(params[:id])
# if book was unique to user
if BookReference.where(book_id: book.id).count == 1
# remove book from system
book.destroy
else
# remove book reference but not book
current_user.books.delete(book)
end
end

Answer 1

rails automatically creates the bookreference for you when you use the has_many through association.
has many through creates a "hidden column" called :book_ids in your user model
so in your form that you want to "save" a book to a user you would use
<%= form_for (#user) do |f| %>
.
.
.
<%= f.label :book_ids %>
<%= f.select :book_ids, Book.all.collect {|x| [x.name, x.id]}, {}, :multiple => true %>
your bookreference should be automatically populated and you can display it in the view. Edit and destroys are automatically done for you too. when you delete or edit it will be only the bookreference table.
if you want to know more about associations you can go to
http://api.rubyonrails.org/
and search for ActiveRecord::Associations

Answer 2

I'm still kinda new to rails as well, but this is what I came up with.
book = user1.books.first
if book.users.size > 1
user1.books.first.destroy
else
book.destroy
end
# in order to delete the book's dependency, you will need
class Book
has_many :bookReferences, :dependent => :destroy
Hope this helps.
NOTE:
When you have a model named like BookReference, you refer the model as :book_reference
In your case, you need to change to
class User
has_many :book_references
has_many :books, through: :book_references
class Book
validates_uniqueness_of :title
has_many :book_references
has_many :users, through: :book_references
class BookReference
belongs_to :user
belongs_to :book
UPDATE:
Since you want the new book created when it's not present in the database, one rails method you could take advantage of is Book.find_or_create(). I'm not going into the details but I feel like later on you would want an autocomplete feature to look up the books.
Take a look at this railscast on Autocomplete. Ryan Bates did a great job on setting up the autocomplete and create new entry if it's not found, which is what you are looking for.
Furthermore, if you need an easier way to allow users to add multiple books, here's another railscast that teaches you about Token Fields