Basically, if one field exists, I want to run a query, otherwise, I want to run a separate one.

`db.Collection.aggregate([
    {
        $match: {
            $cond: [{if fieldOnCollection:{$exists:true}}, {fieldOnCollection:'foo'}, {anotherField:'bar'}]
        }
    }
]);`

I've only ever seen $cond used in other stages and not to construct an actual part of the query consisting of fields on the collection itself

CodePudding user response：

Query1
(without aggregate operators)

Test code here

db.collection.aggregate({
  "$match": {
    "$or": [
      {
        "fieldOnCollection": "foo"
      },
      {
        "$and": [
          {
            "fieldOnCollection": {
              "$exists": false
            }
          },
          {
            "anotherField": "bar"
          }
        ]
      }
    ]
  }
})

Query2
(with aggregate operators similar to your query)

• if field exists, test if its equal with "foo"
• else check if the other field is equal with "bar"

Test code here

db.collection.aggregate([
  {
    "$match": {
      "$expr": {
        "$cond": [
          {
            "$ne": [
              {
                "$type": "$fieldOnCollection"
              },
              "missing"
            ]
          },
          {
            "$eq": [
              "$fieldOnCollection",
              "foo"
            ]
          },
          {
            "$eq": [
              "$anotherField",
              "bar"
            ]
          }
        ]
      }
    }
  }
])

CodePudding user response：

Not clear, what you try to achieve. Anyway, your syntax is wrong. It would be like this:

{ $cond: [
   { $ne : ["$fieldOnCollection", undefined] }, 
   'foo', 
   'bar'
   ]
}

or

{ $cond: {
   if: { $ne : ["$fieldOnCollection", undefined] }, 
   then: 'foo', 
   else: 'bar'
   }
}

If you need to check whether a field exists, see https://stackoverflow.com/a/68255564/3027266