The operator '$ref' is not valid when utilizing $cond in conjunction with a comparison involving a DBRef

My database has a collection with two columns, 'a' and 'b', that can both point to a user. Currently, I am running an aggregate query to match rows where either 'a' or 'b' has a specific value. I want to use $group to group the results based on the column that did not match the input value. The query I tried is as follows:

db.collection.aggregate(
     {'$project': {'a': 1, 'b': 1}},
     {'$group': {'_id': {'$cond': 
            [{'$eq': ['$a', DBRef('users', ObjectId('533af99ca41fd238a4c60f3f'))]},
                 '$b', '$a']}}})

However, this minimal aggregate query fails with the following error message:

Fri Apr 25 01:31:28.140 aggregate failed: {
    "errmsg" : "exception: invalid operator '$ref'",
    "code" : 15999,
    "ok" : 0
} at src/mongo/shell/collection.js:898

It seems like Mongo is interpreting the DBRef in an unexpected way. I have searched for similar issues, but have not found any relevant information.

javascriptmongodbmongodb-query

Answer №1

According to the Aggregation Guide, using DBRef in an aggregation pipeline is not allowed.

Limitations of Aggregation Pipeline

When using the aggregate command for aggregation operations, there are certain restrictions to be aware of.

Type Limitations The aggregation pipeline (page 7) is unable to process values of the following types: Symbol, MinKey, MaxKey, DBRef, Code, and CodeWScope.

http://docs.mongodb.org/manual/MongoDB-aggregation-guide.pdf

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