MongoDB: User defined variable into query

Question

I'm new to MongoDB shell and I need some help !

I have 2 collections: movies and comments. I want to store a value from the second collection and use it to find something in the first.

This is what I have tried :

var result = db.comments.findOne({name: "Arya Stark"})
var movieId = JSON.stringify(result.movie_id.valueOf())
db.movies.findOne({_id: ObjectId($$movieId)})

The above prints that $$movieId is not defined. I have also tried :

db.movies.find({_id: {$toObjectId: movieId}})

and even though my version is 4.2.3, it prints:

unknown operator: $toObjectId.

I have tried everything, I don't know what else to do.

EDIT :

Here's the printed result :

{
        "_id" : ObjectId("5a9427648b0beebeb6957d48"),
        "name" : "Arya Stark",
        "email" : "[email protected]",
        "movie_id" : ObjectId("573a1392f29313caabcd9906"),
        "text" : "Laborum incidunt asperiores accusamus facilis vitae ut quidem. Praesentium provident explicabo odit dolores unde amet architecto. Iure id vero temporibus assumenda eum quia.",
        "date" : ISODate("1978-07-29T23:20:45Z")
}

Basically I want to take the movie_id from the collection comments and use it in the collection movies to search from which movie it is.

Well... I afraid we should have before hand a sample of both collections — felipsmartins
– felipsmartins, Commented Feb 19, 2020 at 22:21

whoami - fakeFaceTrueSoul · Accepted Answer · 2020-02-19 23:50:47Z

1

You don't need to do two database calls, instead use $lookup for JOINS, try to execute below query in shell :

db.comments.aggregate([{ $match: { name: "Arya Stark" } }, {
    $lookup:
    {
        from: "movies",
        localField: "movie_id",
        foreignField: "_id",
        as: "movie"
    }
}, { $unwind: '$movie' }])

Your two issues are :

when you use $$ in your query that respective variable has to be declared locally in query itself.
$toObjectId is an aggregation operator, which can't be used like that in .find().

answered Feb 19, 2020 at 23:50

whoami - fakeFaceTrueSoul

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6 Comments

Mary Voulieri Over a year ago

Thank you for your response! However this is not what I'm trying to do. To be more specific I want to find one random comment from user "Arya Stark", store the movie_id and then use it to find the specific movie from the "movies" collection. The thing is that the _id field accepts values of ObjectId type and I only have the id in string. For example what I have is: "573a1392f29313caabcd9906" but I want it to look like this ObjectId("573a1392f29313caabcd9906"). I can't find a way to pass my variable into the query..

whoami - fakeFaceTrueSoul Over a year ago

@MaryVoulieri : So you mean to say you need random doc from comments collection with specific user ? So for that case add this stage{ $sample: { size: 1 } } in between $match & $lookup. which would get random document for filter { name: "Arya Stark" } & using this query you can do things in one DB call & should work, let me know if doesn't work for you - will update answer..

Mary Voulieri Over a year ago

Thank you very much that's exactly what I wanted to do!

SamuraiJack Over a year ago

What can be used instead of $toObjectId in case of find()?

whoami - fakeFaceTrueSoul Over a year ago

@SamuraiJack : what are you trying to do ? Db version ?

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