ruby-on-rails – 特定类型的多态属性的关联

class Note < ActiveRecord::Base
  # The true polymorphic association
  belongs_to :subject,polymorphic: true

  # Same as subject but where subject_type is 'Volunteer'
  belongs_to :volunteer,source_association: :subject
  # Same as subject but where subject_type is 'Participation'
  belongs_to :participation,source_association: :subject
end

我已经尝试了大量的组合,从阅读关于ApiDock的关联,但没有什么似乎完全正是我想要的.这是我迄今为止最好的

class Note < ActiveRecord::Base
  belongs_to :subject,polymorphic: true
  belongs_to :volunteer,class_name: "Volunteer",foreign_key: :subject_id,conditions: {notes: {subject_type: "Volunteer"}}
  belongs_to :participation,class_name: "Participation",conditions: {notes: {subject_type: "Participation"}}
end

我想通过这个测试：

describe Note do
  context 'on volunteer' do
    let!(:volunteer) { create(:volunteer) }
    let!(:note) { create(:note,subject: volunteer) }
    let!(:unrelated_note) { create(:note) }

    it 'narrows note scope to volunteer' do
      scoped = Note.scoped
      scoped = scoped.joins(:volunteer).where(volunteers: {id: volunteer.id})
      expect(scoped.count).to be 1
      expect(scoped.first.id).to eq note.id
    end

    it 'allows access to the volunteer' do
      expect(note.volunteer).to eq volunteer
    end

    it 'does not return participation' do
      expect(note.participation).to be_nil
    end

  end
end

第一个测试通过,但您不能直接调用该关系：

1) Note on volunteer allows access to the volunteer
     Failure/Error: expect(note.reload.volunteer).to eq volunteer
     ActiveRecord::StatementInvalid:
       PG::Error: ERROR:  missing FROM-clause entry for table "notes"
       LINE 1: ...."deleted" = 'f' AND "volunteers"."id" = 7798 AND "notes"."s...
                                                                    ^
       : SELECT  "volunteers".* FROM "volunteers"  WHERE "volunteers"."deleted" = 'f' AND "volunteers"."id" = 7798 AND "notes"."subject_type" = 'Volunteer' LIMIT 1
     # ./spec/models/note_spec.rb:10:in `block (3 levels) in <top (required)>'

为什么？

我想这样做的原因是因为我正在构建基于解析查询字符串的范围,包括连接到各种模型/ etc;用于构造范围的代码比以上复杂得多 – 它使用collection.reflections等.我目前的解决方案适用于此,但是冒犯了我,我不能直接从注释的实例调用关系.

我可以通过将其分为两个问题来解决这个问题：直接使用范围

scope :scoped_by_volunteer_id,lambda { |volunteer_id| where({subject_type: 'Volunteer',subject_id: volunteer_id}) }
  scope :scoped_by_participation_id,lambda { |participation_id| where({subject_type: 'Participation',subject_id: participation_id}) }

然后只是使用一个吸引器的note.volunteer / note.participation,只是返回note.subject如果它有正确的subject_type(否则为nil),但我认为在Rails必须有一个更好的方法？