Cast of _id in aggregate pipeline

[kunalindoliya]

Mongoose doesn't cast the _id string to ObjectId in $match of aggregation pipeline. Wouldn't it be better to add this functionality ?

[vkarpov15]

The problem is that the aggregation framework can transform the shape of documents, so _id in $match might not be the same type as what the schema defines. We may be able to cast _id in the limited case where $match is the first stage in the aggregation pipeline, but we figure it is a cleaner API to just say Mongoose doesn't cast aggregation pipelines, rather than explain in what cases Mongoose casts aggregation pipelines.