在 MongoDB 表达式中使用 $exists [英] Using $exists in a MongoDB expression

问题描述

对于背景，如果我想比较两个字段，我不能使用以下语法(因为它比较的是文字字符串$lastName"而不是 $lastName 字段的内容):

For background, if I want to compare two fields, I can't use the following syntax (because it compares to the literal string "$lastName" rather than the contents of the $lastName field):

"$match": { "firstName": { $ne : "$lastName"} }

我必须使用这个:

"$match": { "$expr": { $ne : [ "$firstName", "$lastName" ] } }

如果我想测试一个字段是否存在，我必须使用第一种格式:

If I want to test that a field exists I must use the first format:

"$match": { "fullName": { "$exists": true } }

我认为以后一种格式表达 $exists 运算符的正确方法会引发错误:

What I think would be the correct way for expressing the $exists operator in the latter format throws an error:

db.docs.aggregate([
  {
    "$match": { "$expr": { "$exists": [ "$fullName", true ] } }
  }
])
assert: command failed: {
        "ok" : 0,
        "errmsg" : "Unrecognized expression '$exists'",
        "code" : 168,
        "codeName" : "InvalidPipelineOperator"
} : aggregate failed

这是否意味着至少在某些情况下无法组合这些操作?具体来说，假设我想查找 either $fullName $exists OR $firstName $ne $lastName 的文档.可以表达吗?

Does this mean it is not possible to combine these operations, at least in some conditions? Specifically, suppose I want to find docs where either $fullName $exists OR $firstName $ne $lastName. Can that be expressed?

推荐答案

您将需要使用 $or 逻辑运算符来执行此操作.

You will need to use the $or logical operator to do this.

{
   "$or": [
      {
         "$expr": {
            "$ne": [
               "$firstName",
               "$lastName"
            ]
         }
      },
      {
         "fullName": {
            "$exists": true
         }
      }
   ]
}

您上次查询失败，因为 mongod 认为 $exists 是您传递 $expr 运算符的表达式.

You last query failed because mongod thinks $exists is the expression you are passing the the $expr operator.

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