$lookup support for nested localField

[Astro]

Can we use $lookup with nested value for localField?

I have following $lookup stag:

{$lookup:{ from:"users", localField:"appinfo[0].user_ids[0]", foreignField: "_id", as:"userinfo"}}])

where appinfo is a nested array: 

appinfo=[ { name:'xyz', active: true, user_ids=['user1', 'user2', 'user3'] } ]

Any help will be appreciated.

[Asya Kamsky]

Argument to localField must be a string (which is the name of the
field). So you will need to transform (via $project) the value you
need into a field and then use that field name in the $lookup.

Something like this:
[ {$project:{ appinfo_user_id: {$arrayElemAt: [ {$arrayElemAt :
["$appinfo.user_ids,0]}, 0]} }},
{$lookup:{ from:"users", localField:"appinfo_user_id", foreignField:

"_id", as:"userinfo"}} ] )

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Asya Kamsky
Lead Product Manager
MongoDB
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