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.

score 4 · Accepted Answer · answered Jun 23 '17 at 05:05

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'
  }}
}

score 1 · Answer 2 · answered May 20 '19 at 17:58

1

I have a trick for this one:

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

answered May 20 '19 at 17:58

Sunan Jiang

36
3

Dushyant Bangal · Answer 3 · 2020-01-02T11:45:53.327

1

Maybe this was not doable when the question was asked, but now it is.

There is a way to conditionally include/exclude a particular field in $project stage.

This can be achieved using $cond to evaluate your condition, and then return either '$true' or '$false'

{
  myConditionalField:{$cond:{
    if:{
      $eq:['$some_field','some_value']
    },
    then:'$false',
    else:'$true'
  }}
}

Here, the field myConditionalField will be projected only if the value of some_field from previous stage matched some_value.

NOTE: Its assumed here that myConditionalField already exists from a previous stage. If it doesn't, then the else should be $some_field or whatever field you want to be projected.

edited Jan 02 '20 at 11:45

answered Nov 11 '19 at 10:43

Dushyant Bangal

6,048
8
48
80

I guess in the else block maybe it's `$some_field ` rather-than `$true` – whoami - fakeFaceTrueSoul Dec 19 '19 at 16:35
@srinivasy this goes in the `$project` stage. If `myConditionalField` already exists from a previous stage, all you need is `$true`. If it doesn't, then it should be what you specified. – Dushyant Bangal Jan 02 '20 at 11:44

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

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