Conditionally include a field (_id or other) in mongodb project aggregation?

Question

I've got a mongodb aggregation pipeline with a $project stage and I'd like to include certain fields only if conditions are met. Specifically, I'd like to exclude the _id in one condition and include a second field 'second_id' in the other condition.

I know that it's not possible (yet) to exclude fields from a mongodb $project, but is it possible to conditionally include them?

Is there a way to conditionally exclude the _id field? It accepts a 0 or 1, but what if I want to determine that 0 or 1 based on an if statement. How would this be done?

Pseudocode for _id:

$project: { _id: { $ifNull: [ "$user_id", 0 ] } }

The main use of this would be to use the doc.user_id as the result _id, or allow mongodb to create a new autoincrement _id if that user_id is null.

Answer 1

There isn't a way currently to do this within the $project stage, but you can use the $redact stage to remove the field conditionally (i.e. you set the value to 0 like you are doing in your example.

db.collection.aggregate(
  ... matching and stuff ...
  {$project: { _id: { $ifNull: [ "$user_id", 0 ] } }},
  {$redact: {
  {$cond: {
    if: { $eq: [ "$user_id", 0 ] },
    then: '$$PRUNE',
    else: '$$DESCEND'
  }}
}

Answer 2

I have a trick for this one:

"_id": {"$cond": [{"$eq": ["$user_id", null]}, "$nonExistinField", "$user_id"]}