MongoDB聚合中如何将星期、月份转为字符串格式？

要把MongoDB聚合中$dayOfWeek、$month返回的整数转成对应的星期/月份字符串，有几种实用方法，分场景给你说明：

方法1：MongoDB 5.0+ 用$dateToString（最简便） #

MongoDB 5.0新增的$dateToString支持直接格式化日期为星期、月份名称，无需手动映射。常用格式符：

• %a：缩写星期（如"周一"）
• %A：全称星期（如"星期一"）
• %b：缩写月份（如"11月"或"Nov"，取决于数据库语言设置）
• %B：全称月份（如"十一月"或"November"）

针对你的示例数据，聚合代码可写为：

[
  {
    "$project": {
      "_id": 0,
      "weekday_full": {"$dateToString": {"format": "%A", "date": "$timestamp"}},
      "weekday_short": {"$dateToString": {"format": "%a", "date": "$timestamp"}},
      "month_full": {"$dateToString": {"format": "%B", "date": "$timestamp"}},
      "month_short": {"$dateToString": {"format": "%b", "date": "$timestamp"}}
    }
  }
]

中文环境下返回结果：

{
  "weekday_full": "星期一",
  "weekday_short": "周一",
  "month_full": "十一月",
  "month_short": "11月"
}

方法2：低版本MongoDB 用数组映射 #

如果你的MongoDB版本低于5.0，可通过$arrayElemAt配合预设数组实现整数到字符串的映射：

• 注意$dayOfWeek返回值规则：1=周日，2=周一，…，7=周六，需用$subtract减1对应数组下标
• $month返回1-12，对应数组下标0-11

聚合代码示例：

[
  {
    "$project": {
      "_id": 0,
      "weekday": {
        "$arrayElemAt": [
          ["周日", "周一", "周二", "周三", "周四", "周五", "周六"],
          {"$subtract": [{"$dayOfWeek": "$timestamp"}, 1]}
        ]
      },
      "month": {
        "$arrayElemAt": [
          ["一月", "二月", "三月", "四月", "五月", "六月", "七月", "八月", "九月", "十月", "十一月", "十二月"],
          {"$subtract": [{"$month": "$timestamp"}, 1]}
        ]
      }
    }
  }
]

针对你的示例数据，返回结果：

{ "weekday": "周一", "month": "十一月" }

方法3：用$switch自定义映射 #

如果需要更灵活的自定义命名（比如英文缩写、特殊称谓），可以用$switch逐个匹配整数：

[
  {
    "$project": {
      "_id": 0,
      "weekday": {
        "$switch": {
          "branches": [
            {"case": {"$eq": [{"$dayOfWeek": "$timestamp"}, 1]}, "then": "Sunday"},
            {"case": {"$eq": [{"$dayOfWeek": "$timestamp"}, 2]}, "then": "Monday"},
            {"case": {"$eq": [{"$dayOfWeek": "$timestamp"}, 3]}, "then": "Tuesday"},
            {"case": {"$eq": [{"$dayOfWeek": "$timestamp"}, 4]}, "then": "Wednesday"},
            {"case": {"$eq": [{"$dayOfWeek": "$timestamp"}, 5]}, "then": "Thursday"},
            {"case": {"$eq": [{"$dayOfWeek": "$timestamp"}, 6]}, "then": "Friday"},
            {"case": {"$eq": [{"$dayOfWeek": "$timestamp"}, 7]}, "then": "Saturday"}
          ],
          "default": "Unknown"
        }
      },
      "month": {
        "$switch": {
          "branches": [
            {"case": {"$eq": [{"$month": "$timestamp"}, 1]}, "then": "January"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 2]}, "then": "February"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 3]}, "then": "March"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 4]}, "then": "April"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 5]}, "then": "May"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 6]}, "then": "June"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 7]}, "then": "July"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 8]}, "then": "August"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 9]}, "then": "September"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 10]}, "then": "October"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 11]}, "then": "November"},
            {"case": {"$eq": [{"$month": "$timestamp"}, 12]}, "then": "December"}
          ],
          "default": "Unknown"
        }
      }
    }
  }
]

返回结果：

{ "weekday": "Monday", "month": "November" }

内容的提问来源于stack exchange，提问作者yenicead