Rails - Aug 2014 - How avoid concurrency belongs_to and has_many association?

How do I avoid concurrency and keep the data integrity if one user submit a
form before the other?

My issue is that the last user to submit the person/_form.rb will get all
chosen cards, same the cards that first user chose and I want to prohibit
that.

    # person.rb
    class Person < ActiveRecord::Base
      has_many :cards
    end

    # card.rb
    class Card < ActiveRecord::Base
      belongs_to :person
      scope :not_assigned_to_a_person, -> { Card.where(person_id: nil) }
    end

    # create_cards.rb
    create_table :cards do |column|
      column.references :person, index: true
    end

    # person/_form.rb
    = f.input :card_ids,
        collection: Card.not_assigned_to_a_person,
        label_method: :name, value_method: :id,
        input_html: { multiple: true }

StackOverflow:
http://stackoverflow.com/questions/25216219/how-do-i-avoid-concurrency-belongs-to-and-has-many-association
​
-- 
Ricardo do Valle

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