{
  "_id": "doc1", 
    "math": [
      { "examId": 1, "score": 10 },
      { "examId": 2, "score": 20 },
      { "examId": 3, "score": 30 }
    ]  
}
就是向math插入一条{ "examId": 4, "score": 40 }数据