DB rolls back on create action - ruby-on-rails

Question

I'm trying to create a form with a series of checks to prevent duplicates during the simultaneous creation of three model records: one for the parent (assuming it doesn't exist), one for its child (assuming it doesn't exist), and one for a join table between the child and the User (to allow the User to have their own copy of the Song object).
In the current state of the code, The checks seemingly pass, but
the server logs show ROLLBACK, and nothing gets saved
to the database EXCEPT the parent object (artist).
When I try to use the ids of the object, I get the error undefined method id for nil:NilClass, or "couldn't find object without an ID".
The following code is in my controller:
class SongsController < ApplicationController
before_action :authenticate_user!
def create
#artist = Artist.find_by(name: params[:artist][:name].strip.titleize) #look for the artist
#song = Song.find_by(title: params[:artist][:songs_attributes]["0"][:title].strip.titleize)
if #artist.present? && #song.present?
#user_song = current_user.user_songs.find(#song_id)
if #user_song.present?
render html: "THIS SONG IS ALREADY IN YOUR PLAYLIST"
render action: :new
else
#user_song = UserSong.create(user_id: current_user.id, song_id: #song.id)
redirect_to root_path
end
elsif #artist.present? && !#song.present?
#song = #artist.songs.build(title: params[:artist][:songs_attributes]["0"][:title].strip.titleize, lyrics: params[:artist][:songs_attributes]["0"][:lyrics].strip)
#user_song = UserSong.create(user_id: current_user.id, song_id: #song.id)
redirect_to root_path
elsif !#artist.present?
#artist = Artist.create(name: params[:artist][:name].strip.titleize)
#song = #artist.songs.build(title: params[:artist][:songs_attributes]["0"][:title].strip.titleize, lyrics: params[:artist][:songs_attributes]["0"][:lyrics].strip)
#user_song = UserSong.create(user_id: current_user.id, song_id: #song.id)
redirect_to root_path
else
render html: "SOMETHING WENT WRONG. CONTACT ME TO LET ME KNOW IF YOU SEE THIS MESSAGE"
end
end
def index
#songs = Song.all
end
def new
#artist = Artist.new
#artist.songs.build
#user_song = UserSong.new(user_id: current_user.id, song_id: #song_id)
end
def show
#song_id = params["song_id"]
#song = Song.find(params[:id])
end
def destroy
UserSong.where(:song_id => params[:id]).first.destroy
flash[:success] = "The song has been from your playlist"
redirect_to root_path
end
def edit
#song = Song.find(params[:id])
#artist = Artist.find(#song.artist_id)
end
def update
end
private
def set_artist
#artist = Artist.find(params[:id])
end
def artist_params
params.require(:artist).permit(:name, songs_attributes: [:id, :title, :lyrics])
end
def set_song
#song = Song.find(params["song_id"])
end
end
The models:
class Artist < ApplicationRecord
has_many :songs
accepts_nested_attributes_for :songs, reject_if: proc { |attributes| attributes['lyrics'].blank? }
end
class Song < ApplicationRecord
belongs_to :artist
has_many :user_songs
has_many :users, :through => :user_songs
end
class UserSong < ApplicationRecord
belongs_to :song
belongs_to :user
end
Sorry if I haven't abstracted enough. Not really sure how, given that there's no error message, just a rollback (without any validations present in any of the controllers).

Answer 1

Thanks to #coreyward and his pointing out of the fat-model skinny-controller lemma (never knew that was a thing), I was able to cut the code down and arrive at a solution immediately. In my models, I used validates_uniqueness_of and scope in order to prevent duplication of records. In my controller, I used find_or_create_by to seal the deal.
To whom it may concern, the final code is as follows:
class SongsController < ApplicationController
before_action :authenticate_user!
def create
#artist = Artist.find_or_create_by(name: params[:artist][:name].strip.titleize)
#song = #artist.songs.find_or_create_by(title: params[:artist][:songs_attributes]["0"][:title].strip.titleize) do |song|
song.lyrics = params[:artist][:songs_attributes]["0"][:lyrics].strip
end
#user_song = current_user.user_songs.find_or_create_by(song_id: #song.id) do |user_id|
user_id.user_id = current_user.id
end
redirect_to root_path
end
class Song < ApplicationRecord
validates_uniqueness_of :title, scope: :artist_id
belongs_to :artist
has_many :user_songs
has_many :users, :through => :user_songs
end
class Artist < ApplicationRecord
validates_uniqueness_of :name
has_many :songs
accepts_nested_attributes_for :songs, reject_if: proc { |attributes| attributes['lyrics'].blank? }
end
class UserSong < ApplicationRecord
validates_uniqueness_of :song_id, scope: :user_id
belongs_to :song
belongs_to :user
end