问如何使用搜索脚搜索零值？

EN

class Applicant < ActiveRecord::Base
  has_many :trademarks
end

class Trademark < ActiveRecord::Base
  belongs_to :applicant
  searchkick
end

Trademark.where(applicant: nil).count
   (1.7ms)  SELECT COUNT(*) FROM "trademarks" WHERE "trademarks"."applicant_id" IS NULL
 => 1 

# these queries run correctly and show that SearchKick is working
Trademark.search "*" # => 7 records
Trademark.search "*", where: {status: "Removed"} # => 5 records

# When trying to search for applicant == nil, the search fails:

# this returns 7 records, instead of the 1 expected result
Trademark.search "*", where: {applicant: nil} 

# this returns 0 records, instead of the 1 expected result
Trademark.search "*", where: {applicant_id: nil}