You can also use the pipeline stage to perform checks on a sub-docunment array
Here's the example using python (sorry I'm snake people).
db.products.aggregate([
{ '$lookup': {
'from': 'products',
'let': { 'pid': '$products' },
'pipeline': [
{ '$match': { '$expr': { '$in': ['$_id', '$$pid'] } } }
// Add additional stages here
],
'as':'productObjects'
}
])
The catch here is to match all objects in the ObjectId array (foreign _id that is in local field/prop products).
You can also clean up or project the foreign records with additional stages, as indicated by the comment above.