Arrgregate $out mongodb insert for $out failed duplicate Error

Question

Query1: i was trying to combine two collection with selected field of both collections using mongodb Aggregate $lookup as follows

 db.col1.aggregate([
   {
              


        

Answer 1

Your $lookup stage simply returns multiple entries in the user field which then get flattened into individual subdocuments that have the same _id value. You can validate this by replacing the $out stage with a $match instead:

db.col1.aggregate([{
    $match: { _id: ObjectId('5b16305d145a5117552836ec') }
}

The key here is to understand how $unwind works. Imagine you have the following document:

{
    _id: 1,
    users: [ 'a', 'b' ]
}

Once you $unwind this you will get the following two documents:

{
    _id: 1,
    users: 'a'
},
{
    _id: 1,
    users: 'b'
}

So you get the same _id value for two different documents!

Now, when you specify _id: 0 in your projection you remove the _id field from all documents which will cause MongoDB to automatically create new _ids which of course then works.