MongoDB中如何仅更新数组过滤器匹配的第一个元素

我需要仅更新spotsAvailable数组中userId为"empty"的一个子文档。之前的查询会更新所有userId为"empty"的匹配子文档，不符合需求（如下示例）。我试过用聚合并添加limit阶段来仅更新一个元素，但聚合无法修改原始文档。因此只能选择用数组过滤器来仅更新第一个匹配项，这是否可行？我觉得应该有办法只更新数组过滤器中的第一个匹配项，如果没有的话，这绝对是个值得添加的功能。

此代码会更新所有userId为"empty"的对象

const client = await clientPromise;
const db = client.db();
// const query = db.collection('events').aggregate(agg);

const query = await db.collection('events').updateOne({
    _id: new ObjectId("6398c34ca67dbe3286452f23"),
    createdBy: new ObjectId("636c1778f1d09191074f9690"),
    "weights.weight": 12
},
{
    $set: {
        "weights.$.spotsAvailable.$[el2]": {
            "name": "Wayne Wrestler",
            "userId": new ObjectId("636c1778f1d09191074f9690")
        }
    }
},
{
    arrayFilters: [{ "el2": { "userId": "empty" } }]
})

Event： #

{
  "_id": {
    "$oid": "6398c34ca67dbe3286452f23"
  },
  "name": "test",
  "createdBy": {
    "$oid": "636c1778f1d09191074f9690"
  },
  "description": "testing",
  "date": {
    "$date": {
      "$numberLong": "1645488000000"
    }
  },
  "location": {
    "type": "Point",
    "coordinates": [
      0,
      0
    ]
  },
  "weights": [
    {
      "spotsAvailable": [
        {
          "name": "empty",
          "userId": "empty"
        },
        {
          "name": "empty",
          "userId": "empty"
        },
        {
          "name": "empty",
          "userId": "empty"
        }
      ],
      "weight": 12
    },
    {
      "spotsAvailable": [
        {
// 本应仅更新其中一个，但两个都被更新了
          "name": "Wayne Wrestler",
          "userId": {
            "$oid": "636c1778f1d09191074f9690"
          }
        },
        {
          "name": "Wayne Wrestler",
          "userId": {
            "$oid": "636c1778f1d09191074f9690"
          }
        }
      ],
      "weight": 15
    }
  ],
  "eventApplicants": [
    {
      "userId": {
        "$oid": "636c1778f1d09191074f9690"
      },
      "name": "Wayne Wrestler",
      "weight": 12
    }
  ]
}

User： #

{
  "_id": {
    "$oid": "636c1778f1d09191074f9690"
  },
  "name": "Wayne Wrestler",
  "email": "wakywayne80@gmail.com",
  "image": "https://lh3.googleusercontent.com/a/ALm5wu32gXjDIRxncjjQA9I4Yl-sjFH5EWsTlmvdM_0kiw=s96-c",
  "emailVerified": {
    "$date": {
      "$numberLong": "1670864727212"
    }
  },
  "createdEvents": [
    {
      "createdEventName": "test",
      "createdEventDate": {
        "$date": {
          "$numberLong": "1645488000000"
        }
      },
      "createdEventDescription": "testing",
      "createdEventWeights": [
        {
          "weight": "12",
          "filled": [
            false,
            false,
            false
          ]
        },
        {
          "weight": "15",
          "filled": [
            false,
            false
          ]
        }
      ],
      "createdEventId": {
        "$oid": "6398c34ca67dbe3286452f23"
      }
    }
  ],
  "userSignedUpEvents": [],
  "availableWeights": [
    1,
    123
  ],
  "signedUpEvents": [
    {
      "eventId": {
        "$oid": "636c722f67642c30dc5ffc30"
      },
      "eventName": "Utah",
      "eventDate": {
        "$date": {
          "$numberLong": "1667913330000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "636c722f67642c30dc5ffc30"
      },
      "eventName": "Utah",
      "eventDate": {
        "$date": {
          "$numberLong": "1667913330000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "637ec484ac2d675b30590b47"
      },
      "eventName": "Maybe?",
      "eventDate": {
        "$date": {
          "$numberLong": "1672272000000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "636c722f67642c30dc5ffc30"
      },
      "eventName": "Utah",
      "eventDate": {
        "$date": {
          "$numberLong": "1667913330000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "638d5274628db2a7bf61df49"
      },
      "eventName": "Eva's",
      "eventDate": {
        "$date": {
          "$numberLong": "1698019200000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "636c722f67642c30dc5ffc30"
      },
      "eventName": "Utah",
      "eventDate": {
        "$date": {
          "$numberLong": "1667913330000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "6398a922abb5c168ede595fb"
      },
      "eventName": "Nikko's event",
      "eventDate": {
        "$date": {
          "$numberLong": "1670976000000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "6398a922abb5c168ede595fb"
      },
      "eventName": "Nikko's event",
      "eventDate": {
        "$date": {
          "$numberLong": "1670976000000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "6398c34ca67dbe3286452f23"
      },
      "eventName": "test",
      "eventDate": {
        "$date": {
          "$numberLong": "1645488000000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "6398c34ca67dbe3286452f23"
      },
      "eventName": "test",
      "eventDate": {
        "$date": {
          "$numberLong": "1645488000000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "6398c34ca67dbe3286452f23"
      },
      "eventName": "test",
      "eventDate": {
        "$date": {
          "$numberLong": "1645488000000"
        }
      },
      "accepted": false
    },
    {
      "eventId": {
        "$oid": "6398c34ca67dbe3286452f23"
      },
      "eventName": "test",
      "eventDate": {
        "$date": {
          "$numberLong": "1645488000000"
        }
      },
      "accepted": false
    }
  ]
}

• 不使用new ObjectId语法传入变量
• 使用new ObjectId语法传入变量
• 使用从Compass复制到Node驱动的聚合代码中的硬编码值

以上方法要么无效，要么会导致所有userId为"empty"的子文档被更新

直接使用数组过滤器无法实现仅更新第一个匹配的子文档，不过可以通过以下两种方式达成需求：

方法一：聚合管道单步更新（MongoDB 4.2+支持） #

利用聚合管道的表达式精准定位第一个空元素，一次性完成更新：

const query = await db.collection('events').updateOne(
  {
    _id: new ObjectId("6398c34ca67dbe3286452f23"),
    createdBy: new ObjectId("636c1778f1d09191074f9690"),
    "weights.weight": 12
  },
  [
    {
      $set: {
        weights: {
          $map: {
            input: "$weights",
            as: "w",
            in: {
              $cond: [
                { $eq: ["$$w.weight", 12] },
                {
                  ..."$$w",
                  spotsAvailable: {
                    $map: {
                      input: "$$w.spotsAvailable",
                      as: "s",
                      in: {
                        $cond: [
                          { $and: [
                            { $eq: ["$$s.userId", "empty"] },
                            { $eq: [{ $indexOfArray: ["$$w.spotsAvailable.userId", "empty"] }, { $indexOfArray: ["$$w.spotsAvailable", "$$s"] }] }
                          ]},
                          {
                            "name": "Wayne Wrestler",
                            "userId": new ObjectId("636c1778f1d09191074f9690")
                          },
                          "$$s"
                        ]
                      }
                    }
                  }
                },
                "$$w"
              ]
            }
          }
        }
      }
    }
  ]
)

通过$indexOfArray获取第一个userId为"empty"的元素索引，仅更新该索引对应的子文档。

方法二：先查索引再更新 #

分两步操作，先获取第一个空元素的索引，再通过索引直接定位更新：

// 第一步：查询获取第一个empty元素的索引
const event = await db.collection('events').findOne(
  {
    _id: new ObjectId("6398c34ca67dbe3286452f23"),
    createdBy: new ObjectId("636c1778f1d09191074f9690"),
    "weights.weight": 12
  },
  {
    projection: { "weights.$.spotsAvailable": 1 }
  }
);

const firstEmptyIndex = event.weights[0].spotsAvailable.findIndex(s => s.userId === "empty");

if (firstEmptyIndex !== -1) {
  // 第二步：通过索引精准更新
  const updateResult = await db.collection('events').updateOne(
    {
      _id: new ObjectId("6398c34ca67dbe3286452f23"),
      createdBy: new ObjectId("636c1778f1d09191074f9690"),
      "weights.weight": 12
    },
    {
      $set: {
        [`weights.$.spotsAvailable.${firstEmptyIndex}`]: {
          "name": "Wayne Wrestler",
          "userId": new ObjectId("636c1778f1d09191074f9690")
        }
      }
    }
  );
}

这种方式逻辑直观，适合对聚合管道不熟悉的场景。

内容的提问来源于stack exchange，提问作者Wayne