Spring MongoDB聚合查询无法获取指定字段重复值列表求助

问题场景 #

现有如下MongoDB数据集：

[
  {
    "_id": 1,
    "domainName": "test1.com",
    "hosting": "hostgator.com",
    "to": "nanjing",
    "travelDate": "09/06/2022",
    "trainNumber": 123
  },
  {
    "_id": 2,
    "domainName": "test2.com",
    "hosting": "aws.amazon.com",
    "to": "beijing",
    "travelDate": "09/02/2022",
    "trainNumber": 123
  },
  {
    "_id": 3,
    "domainName": "test3.com",
    "hosting": "aws.amazon.com"
  },
  {
    "_id": 4,
    "domainName": "test4.com",
    "hosting": "hostgator.com",
    "to": "shanghai",
    "travelDate": "09/06/2022",
    "trainNumber": 1234
  },
  {
    "_id": 5,
    "domainName": "test5.com",
    "hosting": "aws.amazon.com",
    "to": "wuhan",
    "travelDate": "09/06/2022",
    "trainNumber": 1234
  },
  {
    "_id": 6,
    "domainName": "test6.com",
    "hosting": "cloud.google.com",
    "to": "nanjing",
    "travelDate": "09/06/2022",
    "trainNumber": 123
  },
  {
    "_id": 7,
    "domainName": "test7.com",
    "hosting": "aws.amazon.com",
    "to": "nanjing",
    "travelDate": "09/02/2022",
    "trainNumber": 123
  },
  {
    "_id": 8,
    "domainName": "test8.com",
    "hosting": "hostgator.com",
    "to": "nanjing",
    "travelDate": "09/02/2022",
    "trainNumber": 123
  },
  {
    "_id": 9,
    "domainName": "test9.com",
    "hosting": "cloud.google.com",
    "to": "nanjing",
    "travelDate": "09/02/2022",
    "trainNumber": 123
  },
  {
    "_id": 10,
    "domainName": "test10.com",
    "hosting": "godaddy.com",
    "to": "nanjing",
    "travelDate": "09/02/2022",
    "trainNumber": 123
  }
]

使用原生MongoDB聚合查询，可正确获取符合条件的to字段列表（包含重复值）：

db.collection.aggregate({
  $match: {
    travelDate: {
      $eq: "09/06/2022"
    },
    trainNumber: {
      $eq: 1234
    }
  }
},
{
  $group: {
    _id: "",
    destStations: {
      $push: "$to"
    }
  }
},
{
  $project: {
    _id: 0
  }
})

预期结果：

[
  {
    "destStations": [
      "shanghai",
      "wuhan"
    ]
  }
]

但使用以下Spring MongoDB聚合代码无法得到正确结果：

Aggregation agg = newAggregation(                
    match(Criteria.where("travelDate").is(travelDate).and("trainNumber").is(trainNumber)),
    group("id").push("to").as("destStations"),
    project().andExclude("id")
);

错误原因 #

1. 分组逻辑不匹配：原生查询中$group的_id设为空字符串，会将所有匹配$match条件的文档合并为一个分组；而Spring代码中group("id")实际是按文档的_id字段分组，导致每个匹配文档单独成组，无法得到合并后的列表。
2. 投影排除字段错误：原生查询排除的是MongoDB默认主键_id，但Spring代码中andExclude("id")对应的是不存在的id字段，导致最终结果仍会包含分组后的_id。

修正后的代码 #

将分组条件改为空字符串，并修正投影排除的字段：

Aggregation agg = newAggregation(                
    match(Criteria.where("travelDate").is(travelDate).and("trainNumber").is(trainNumber)),
    group("").push("to").as("destStations"),
    project().andExclude("_id")
);

验证结果 #

修正后的代码执行后，将返回与原生查询完全一致的结果：

[
  {
    "destStations": [
      "shanghai",
      "wuhan"
    ]
  }
]

内容的提问来源于stack exchange，提问作者SSM Tariq