Database
[{_id:ObjectId("6162883719592ea3350d3c87"),fullName:"Random User",username:"ruser1",password:"asdadasd",portfolio:[{equity:[]},{crypto:[]},{etf:[]},{cash:[]}]}]
Query
db.collection.update({_id:ObjectId("6162883719592ea3350d3c87")},{$push:{"portfolio.$[portfolio].equity":{name:"AAPL",price:100,quantity:1}}},{arrayFilters:[{"portfolio.equity":{$exists:true}}]})