Mongo Aggregate Conditional query within $match

Question

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

what is foo? a driver variable(for example a javascript variable)? or a value that must be calculated in the database?(for example a mongodb field or variable `$$var`) — Takis, Oct 01 '21 at 17:09
Apologies for unclarity. `foo` was supposed to represent just a generic boolean for the `$cond`. But I updated the post. The boolean itself is for if a specific field exists — user2402616, Oct 01 '21 at 17:18

Takis · Accepted Answer · 2021-10-01T18:02:01.543

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"
            ]
          }
        ]
      }
    }
  }
])

score 0 · Answer 2 · answered Oct 01 '21 at 17:30

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

Mongo Aggregate Conditional query within $match

2 Answers2