$in requires an array as a second argument [duplicate]

Question

I'm trying to make some aggregations and I have the following problem. I need to use "pipeline" and I'm getting an error when the array I'm looking for is missing.

{
    $lookup: {
        from: 'comments',
        let: { comments: '$comments' },
        pipeline: [
            {
                $match: {
                    $expr: {
                        $in: ['$_id', '$$comments']
                    },
                    isDeleted: false
                }
            }
        ],
        as: 'comments'
    }
}

With this stage I get the following error:

'$in requires an array as a second argument, found: missing'

Because not all documents have the field "comments".

Note: I'm using pipeline instead of foreingField and localField because I need to filter with isDeleted: false and maybe other matching conditions.

Is there anyway to make this lookup only if the document has the field comments?

Thank you!

Answer 1

you can add $ifNull or $and with not null or exists condition in $expr

{
    $lookup: {
        from: 'comments',
        let: { comments: '$comments' },
        pipeline: [
            {
                $match: {
                    $expr: {$and: [
                        {$in: ['$_id', {$ifNull :['$$comments',[]]}]},
                        {$eq: ["$isDeleted", false]}
                    ]}
                }
            }
        ],
        as: 'comments'
    }
}