在字段上使用$ dateFromString时，避免结果中包含空字段 [英] Avoid having null fields in result when using $dateFromString on a field

问题描述

如果某些文档中的字段为null，如何在字段上使用$ dateFromString时避免结果为空字段.

How to avoid having null fields in result when using $dateFromString on a field if the field is null in some documents.

Mongo Playground -结果中的日期字段不应为空

Mongo Playground - the result should not have any date fields with null

文档:

[
  {
    "date1": ISODate("2019-05-20T19:04:35.000Z")
  },
  {
    "date1": ISODate("2019-05-20T19:04:35.000Z"),
    "date2": ISODate("2019-05-20T19:04:35.000Z")
  },
  {
    "date3": ISODate("2019-05-20T19:04:35.000Z")
  }
]

预期:

[
  {
    "date1": ISODate("2019-05-20T00:00:00Z")
  },
  {
    "date1": ISODate("2019-05-20T00:00:00Z"),
    "date2": ISODate("2019-05-20T00:00:00Z")
  },
  {
    "date3": ISODate("2019-05-20T00:00:00Z")
  }
]

推荐答案

您可以使用 $ type 检查日期是否为空或缺少日期，如果缺少则请使用 $$ REMOVE ，

You can do it with checking if date is null or missing filed using $type, if it is missing then remove using $$REMOVE,

db.collection.aggregate([
  {
    $project: {
      date1: {
        $cond: {
          if: {
            $or: [
              { $eq: [{ $type: "$date1" }, "missing"] },
              { $eq: ["$date1", null] }
            ]
          },
          then: "$$REMOVE",
          else: {
            $dateFromString: {
              dateString: {
                $dateToString: {
                  format: "%Y-%m-%d",
                  date: "$date1"
                }
              }
            }
          }
        }
      },
      date2: {
        $cond: {
          if: {
            $or: [
              { $eq: [{ $type: "$date2" }, "missing"] },
              { $eq: ["$date2", null] }
            ]
          },
          then: "$$REMOVE",
          else: {
            $dateFromString: {
              dateString: {
                $dateToString: {
                  format: "%Y-%m-%d",
                  date: "$date2"
                }
              }
            }
          }
        }
      },
      date3: {
        $cond: {
          if: {
            $or: [
              { $eq: [{ $type: "$date3" }, "missing"] },
              { $eq: ["$date3", null] }
            ]
          },
          then: "$$REMOVE",
          else: {
            $dateFromString: {
              dateString: {
                $dateToString: {
                  format: "%Y-%m-%d",
                  date: "$date3"
                }
              }
            }
          }
        }
      }
    }
  }
])

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