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Using find_or_create_by with accepts_nested_attributes_for

Rails • Asked by Nino Rosella

Hi gang...

I have a Book which has_many :authors, through: :book_authors

Instead of creating a new Author each time I create a book I'd like to use find_or_create_by on the author's name attribute. Really struggling to work out where this code should go. Using cocoon gem for the form.

Here's what I currently have that works.

class Book < ApplicationRecord

has_many :book_authors
has_many :authors, through: :book_authors
belongs_to :user

accepts_nested_attributes_for :authors, allow_destroy: true

class Author < ApplicationRecord
has_many :book_authors
has_many :books, through: :book_authors

class BookAuthor < ApplicationRecord
belongs_to :book
belongs_to :author

class BooksController < ApplicationController
before_action :authenticate_user!

def new
@book =

def create
@book = current_user.books.create(book_params)
@book.authors.each {|author| author.user_id =}

    redirect_to book_path(@book)
    render :new


def book_params
params.require(:book).permit(:title, authors_attributes: [:id, :name, :_destroy])

<%= simple_form_for @book do |f| %>
<%= f.input :title %>
<div id='authors'>
<%= f.simple_fields_for :authors do |author| %>
<%= render 'author_fields', :f => author %>
<% end %>
<div class='links'>
<%= link_to_add_association 'Add another author', f, :authors %>
<% end %>

<div class="nested-fields">
<%= f.input :name, label: "Author(s)", collection: @authors, value_method: :name, input_html: {value: @authors, class: 'new-author'} %>
<%= link_to_remove_association "Remove this author", f %>

Don't suppose anyone could lend a hand before this drives me insane..? :s

Hey Nino, 

I haven't tested this, but I believe this should work for you on your create method:

@book = current_user.books.create(book_params)

author = Author.find_or_create_by(name: "foo") do |author|
  # do stuff if author is new


find_or_create_by will either create and return a new object or return the found object. So once you have the author object, just create the @book.book_authors record directly.

Hi Jacob,

That's been a massive help, so thanks. I just have one small problem now.

When I save the Book and then do


I have two records saved. One is the correct record that was retrieved by the find_or_create_by method. The second record is new and I believe this is being created by the first line of code:

@book = current_user.books.create(book_params)

Is there a way to skip the saving of the new record?

Oh, duh I didn't pay attention to your book_params, it includes authors_attirbutes.

If this were my project, right or wrong, I'd make two sets of params, one for author and one for book. So something like:

def book_params

def author_params
  params.require(:book).permit(authors_attributes: [:id, :name, :_destroy])

This way you can create your book without the rails magic also creating the association when you create the book when passing book_params.

There could be a better way to handle this scenario that rails provides, but I haven't found it so I usually resort to this sort of setup to get the job done.

Jacob, you nailed it.

Thanks so much for your help!

Woohoo, glad that worked for you!

Good luck!

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