Database
db={"products":[{"_id":{"$oid":"652f3470729e8d8a59b221dc"},"title":"Hat","category":{"$oid":"652af415729e8d8a59b2202a"}},{"_id":{"$oid":"652f3470729e8d8a59b221dd"},"title":"Hat","category":{"$oid":"652af415729e8d8a59b2202b"}}],"categories":[{"_id":{"$oid":"652af415729e8d8a59b2202a"},"name":"Womens"},{"_id":{"$oid":"652af415729e8d8a59b2202b"},"name":"Mens"}]}
Query
db.products.aggregate([{$match:{"title":"Hat"}},{$lookup:{from:"categories",localField:"category",foreignField:"_id",as:"category"}},{$match:{"category.name":"Mens"}}])