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Rails HABTM checkbox validation
model Profile
  has_and_belongs_to_many :browsers
end

model Browser
  has_and_belongs_to_many :profiles
end

validate :must_have_at_least_one_browser

private

  def must_have_at_least_one_browser
    if browsers.blank?
      errors.add(:base, 'You must select at least one browser')
    end
  end

def must_have_at_least_one_browser
    errors.add(:base, 'You must select at least one browser') unless browser_ids.count > 0
  end