ElasticSearch-Rails multi_match字段

Question

我的模型关联：

class CertificatePeriod < ActiveRecord::Base
  has_many :certificate_orders
  has_many :order_transactions, through: :certificate_orders
end

class CertificateOrder < ActiveRecord::Base
  belongs_to :user
  belongs_to :certificate_period
  has_one :order_transaction
end

class OrderTransaction < ActiveRecord::Base
  belongs_to :certificate_order
end

在OrderTransaction模型中：

class OrderTransaction < ActiveRecord::Base
  def as_indexed_json(options={})
    self.as_json(only: [:id, cardholdername, :userid],
    include: {certificate_order: {include: {user: {only: [:first_name, :last_name], methods: :full_name}}}})
  end
end

此索引的示例响应：

{"id"=>7235,
 "cardholdername"=>"NUR SARUHAN",
 "userid"=>"zzzyyyxxxx",
 "certificate_order"=>
  {"id"=>292,
   "user_id"=>13186,
   "certificate_enrollment_id"=>768,
   "slug"=>"eafe0be5-0b1f-4e4f-8929-d3f9330a4a86f7c25b1199dcd5",
   "created_at"=>Mon, 11 Apr 2016 18:30:58 EEST +03:00,
   "updated_at"=>Mon, 11 Apr 2016 18:30:58 EEST +03:00,
   "user"=>{"first_name"=>"Nur", "last_name"=>"SARUHAN", "full_name"=>"NUR SARUHAN"}}}

我使用过类似这样的搜索查询：

query: {
  bool: {
    must: [
      {ids: {values: @certificate_period.order_transactions.pluck(:id)}},
      {bool: {
        should: [
          {multi_match: {query: params[:q], fields: ['cardholdername', 'certificate_order.user.first_name']}}
        ]
      }}
    ]
  }
}).records.to_a

我的问题是：

实际上，这个搜索查询是有效的，但是以下几行返回的是完全相同的记录。

{multi_match: {query: params[:q], fields: ['cardholdername', 'certificate_order.user.first_name']}}

和

{multi_match: {query: params[:q], fields: ['cardholdername']}}

如果我只是在multi_match字段中使用certificate_order.user.first_name作为参数，它将返回一个空数组。所以没有certificate_order.user.first_name的影响。我已经用谷歌搜索了3天，但我没有找到正确的答案。如何获取certificate_order.user.first_name参数的值？