Add a custom validation in rails 3 - ruby-on-rails-3

Question

I am working with a nested model a question has multiple answers and only one can be marked as correct how can i validate to check that only one question was marked as correct. The correct is a boolean field.
#question model
validate :one_correct_answers
def one_correct_answers
if self.choices.correct_choices > 1
errors.add(:base, "please select only one correct answer")
end
end

Answer 1

In Question model
class Question
has_many :choices
accepts_nested_attributes_for :choices, :reject_if => ->(choice){ choice[:value].blank? }
validate :only_one_correct_answer
private
def only_one_correct_answer
unless (choices.select{ |choice| choice.correct }.size == 1)
errors.add(:choices, "You must provide only 1 correct answer")
end
end
end
In Form HTML
<input name="question[choices_attributes][0][correct]" type="checkbox">
<input name="question[choices_attributes][1][correct]" type="checkbox">
<input name="question[choices_attributes][2][correct]" type="checkbox">
<input name="question[choices_attributes][3][correct]" type="checkbox"> ... till n
and in QuestionsController
#question = Question.new(params[:question])
#question.valid? => will automatically call Question#only_one_correct_answer and add errors,if any.
I hope, this will help you. :)

Answer 2

First, your checkboxes should look like this:
<input id="something_" name="something[]" type="checkbox" value="<%= some_id %>">
This way, when you submit the form, the params should look like and array of the checked checkboxes:
params: something => [1, 2]
Then, on your controller you set the variable correct_choices to this array, and validate with your custom validator.