Rails 3：通过关联项验证至少一个有多个的存在

我有两个模型：Project and ProjectDiscipline:

class Project < ActiveRecord::Base
  has_many :project_disciplinizations, :dependent => :destroy
  has_many :project_disciplines, through: :project_disciplinizations
  attr_accessible :project_discipline_ids
  attr_accessible :project_disciplines_attributes
  accepts_nested_attributes_for :project_disciplines, :reject_if => proc { |attributes| attributes['name'].blank? }
end

class ProjectDiscipline < ActiveRecord::Base
  attr_accessible :name
  has_many :project_disciplinizations, :dependent => :destroy
  has_many :projects, through: :project_disciplinizations
end

class ProjectDisciplinization < ActiveRecord::Base
  attr_accessible :project_discipline_id, :project_id
  belongs_to :project_discipline
  belongs_to :project
end

关于新建/编辑Project表单中，我有一个学科列表和每个学科的复选框，以便用户可以选择学科：

<div class="control-group">
  <label class="control-label">Check disciplines that apply</label>
  <div class="controls">
    <%= f.collection_check_boxes(:project_discipline_ids, ProjectDiscipline.order('name'), :id, :name, {}, {:class => 'checkbox'}) {|input| input.label(:class => 'checkbox') { input.check_box + input.text }} %>
    <p class="help-block">You must choose at least one discipline.</p>
  </div>
</div>

我想添加一项验证，要求至少检查一门学科。我已经尝试过，但我还没弄清楚。我如何添加此验证？

答案之前的旁注，根据模型的结构，我将使用 has_and_belongs_to_many 而不是使用此显式链接模型，因为链接模型似乎没有添加任何有价值的内容。

不管怎样，答案是相同的，那就是使用自定义验证。根据您是按原样处理还是简化为 has_and_belongs_to 许多，您需要稍微不同地进行验证。

validate :has_project_disciplines

def has_project_disciplines
  errors.add(:base, 'must add at least one discipline') if self.project_disciplinizations.blank?
end

或使用 has_and_belongs_to_many

validate :has_project_disciplines

def has_project_disciplines
  errors.add(:base, 'must add at least one discipline') if self.project_disciplines.blank?
end