How to update nested data in Rails - ruby-on-rails-3

Question

I have some users who take quizzes. I track the result they chose. I need to figure out how to allow them to change their quiz submission. If I just associate the answer, they'll have answered the question twice. Building up the data is complex, does ActiveRecord provide a way to deal with this?
This whole example will run in a standalone file.
Here is my schema:
require 'active_record'
ActiveRecord::Base.establish_connection adapter: 'sqlite3', database: ':memory:'
ActiveRecord::Schema.define do
self.verbose = false
create_table :users do |t|
t.string :name
end
create_table :questions do |t|
t.string :name
end
create_table :question_results do |t|
t.string :name
t.integer :question_id
end
create_table :question_results_users do |t|
t.integer :user_id
t.integer :question_result_id
end
end
here are my models:
class User < ActiveRecord::Base
has_and_belongs_to_many :question_results
has_many :questions, through: :question_results
end
class Question < ActiveRecord::Base
has_many :question_results
end
class QuestionResult < ActiveRecord::Base
belongs_to :question
has_and_belongs_to_many :users
end
Lets make three questions and some answers:
Question.create! name: "What's your favourite movie?" do |question|
question.question_results.build name: 'Gattaca'
question.question_results.build name: 'Super Troopers'
end
Question.create! name: 'Who do you want to be president?' do |question|
question.question_results.build name: 'Barack Obama'
question.question_results.build name: 'Mitt Romney'
question.question_results.build name: 'Mickey Mouse'
end
Question.create! name: "What's your favourite colour?" do |question|
question.question_results.build name: 'black'
question.question_results.build name: 'green'
end
Lets make a user:
jim = User.create! name: 'Jim'
jim.question_results << QuestionResult.find_by_name('Gattaca')
jim.question_results << QuestionResult.find_by_name('Barack Obama')
jim.question_results << QuestionResult.find_by_name('black')
jim.question_results # => [#<QuestionResult id: 1, name: "Gattaca", question_id: 1>, #<QuestionResult id: 3, name: "Barack Obama", question_id: 2>, #<QuestionResult id: 6, name: "black", question_id: 3>]
jim.question_results.map(&:question) # => [#<Question id: 1, name: "What's your favourite movie?">, #<Question id: 2, name: "Who do you want to be president?">, #<Question id: 3, name: "What's your favourite colour?">]
Now Jim changes his mind, he decides he likes Super Troopers and doesn't want to vote, but he still likes black. How do I update this without having him answer the question multiple times?
# pretend we're in a controller here (also note that I can change the format of the data, if there is something more convenient)
posted_from_form = {
questions: {
Question.all[0].id => 'Super Troopers',
Question.all[1].id => '',
Question.all[2].id => 'black',
}
}

Answer 1

First lets change the format of our data so that the results are in id form instead of name form:
posted_from_form = {
questions: {
Question.all[0].id => QuestionResult.find_by_name('Super Troopers').id.to_s,
Question.all[1].id => '',
Question.all[2].id => QuestionResult.find_by_name('black').id,
}
}
Then you can set the ids directly, and ActiveRecord will handle all of the complexity:
jim.question_result_ids = posted_from_form[:questions].values # => ["2", "", 6]
jim.question_results # => [#<QuestionResult id: 6, name: "black", question_id: 3>, #<QuestionResult id: 2, name: "Super Troopers", question_id: 1>]