ActiveRecord::HasManyThroughNestedAssociationsAreReadonly Exception: Cannot modify association - ruby-on-rails

Question

NOTE:
Yes, I know that there is supposedly duplicate posts on this matter, but the latest one that I saw is from 10 years ago (Link to that post), was never solved, and none of the posts had gotten an answer that could solve my issue, thus here I am...
I have been trying to make an mock website where an User can make Binders, that hold Decks, which hold Flashcards.
My problem here is (most likely) with my joins table, the Flashcard. Flashcard is my joins table because I am trying to also create an page where an User can browse through all of the Flashcards made by every User, and add it to one of their Decks (basically, shared access).
Every time I try to create an Flashcard, I get this error:
ActiveRecord::HasManyThroughNestedAssociationsAreReadonly Exception: Cannot modify association 'User#flashcards' because it goes through more than one other association.
Here is my code:
User model:
class User < ApplicationRecord
has_secure_password
has_many :binders
has_many :decks, through: :binders
has_many :flashcards, through: :decks
end
Binder model:
class Binder < ApplicationRecord
belongs_to :user
has_many :decks
has_many :flashcards, through: :decks
end
Deck model:
class Deck < ApplicationRecord
belongs_to :binder
has_many :flashcards
end
Flashcard model:
class Flashcard < ApplicationRecord
belongs_to :user
belongs_to :deck
validates :user, :deck, presence: true
end
flashcards_controller.rb:
class FlashcardsController < ApplicationController
def create
flashcard = set_user.flashcards.new(flashcard_params)
flashcard.user_id = user.id
if flashcard.save
render json: flashcard, status: :created
else
render json: {errors: flashcard.errors.full_messages}, status: :unauthorized
end
end
private
def flashcard_params
params.require(:flashcard).permit(:id, :user_id, :deck_id, :question, :answer) # Verify if the user_id is needed from frontend
end
end
My schema.rb:
create_table "binders", force: :cascade do |t|
t.string "name"
t.integer "user_id"
end
create_table "decks", force: :cascade do |t|
t.integer "binder_id"
t.string "name"
end
create_table "flashcards", force: :cascade do |t|
t.integer "user_id"
t.integer "deck_id"
t.string "question"
t.string "answer"
end
create_table "users", force: :cascade do |t|
t.string "username"
t.string "password_digest"
end

Answer 1

Okay, so I figured out how to fix this issue...
My updated flashcards_controller.rb create method:
def create
deck = current_user.decks.find_by(id: params[:deck_id])
flashcard = deck.flashcards.new(flashcard_params)
flashcard.user_id = current_user.id
if flashcard.save
render json: flashcard, status: :created
else
render json: {errors: flashcard.errors.full_messages}, status: :unauthorized
end
end
This new create method works, because after the Flashcard object is created (but before attempting to save it), the method will keep checking for the new Flashcard object until it is found, assigns the user_id to it, and then attempts to save the new object...