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I have 2 collections: Movie and User. in Movie I saved information about movies such as stars who actors that play in movie. i saved this field as an array in Movie collection. now I want to write a query that return stars name. but this name field saved in User collection. how can I extract data from another collection to this collection? I write this function but it's wrong and stars_doc is empty.
this is my function :

async function starsActMostMovies()
const res = await Movie.aggregate([
 
 $unwind: '$stars'
 
 ,
 
 $lookup:
 
 from: "User",
 localField: "_id",
 foreignField : "stars",
 as: "stars_doc"
 
 
 ,
 
 $group: 
 _id : '$stars' ,
 count : $sum : 1
 
 
 ,
 $sort: count: -1
])
return res

starsActMostMovies().then(function(result)
 console.log(result))

and in this link I wrote my database models.

Looking at your database models I think you may have the incorrect ref string in your movie schema definition. Inside of your movie schema you have:

stars:[
type: Schema.Types.ObjectId,
ref: 'userSchema'
],

but your user model is defined as:

var User = mongoose.model('user', userSchema);

The ref in your movie schema needs to refer to the name you give the user model which is in this case is 'user' not userSchema