可能需要使用 map-reduce 完成，这种事情不是 nosql 的强项。

def group_by(field)
  map = %Q{
    function() {
      emit(this.#{field}, { count: 1 });
    }
  }

  reduce = %Q{
    function(key, values) {
      var result = { count: 0 };
      values.forEach(function(value) {
        result.count += value.count;
      });
      return result;
    }
  }

  result = self.map_reduce(map, reduce).out(inline: true).to_a

  hash = {}
  result.each do |r|
    hash[r["_id"]] = r["value"]["count"].to_i
  end

  #hash.sort_by{|_k, v| v}.to_h
  #Hash[*hash.sort_by{|_k, v| v}.flatten]
  hash.sort_by(&:last).to_h
end

#1 楼 @debugger 我遇到failed with error 16052: "exception: could not create cursor over th_test.messages for query : {} sort : { created_at: -1 }"这个错误，未找到解决方案请教一下是哪里有错？

def messages
  map = %Q{
    function() {
      emit(this.sr_flag, { count: 1 });
    }
  }

  reduce = %Q{
    function(key, values) {
      var result = { count: 0 };
      values.forEach(function(value) {
        result.count += value.count;
      });
      return result;
    }
  }

  result = Message.map_reduce(map, reduce).out(inline: true).to_a
  result
  # Message.or(receiver_document_id: "#{self.id}", :recipient_delete.ne=>true).or(sender_document_id: "#{self.id}", :sender_delete.ne=>true).desc(:created_at)
end