Database
db={"stocks":[{symbol:"aapl",name:"Apple Inc."},{symbol:"googl",name:"Alphabet Inc."},{symbol:"amzn",name:"Amazon.com, Inc."}],"prices":[{index_id:"AAPL",price:145.44},{index_id:"GOOGL",price:2672.75},{index_id:"MSFT",price:237.58}]}
Query
db.stocks.aggregate([{$addFields:{symbol_upper:{$toUpper:"$symbol"}}},{$lookup:{from:"prices",localField:"symbol_upper",foreignField:"index_id",as:"foo"}},{$project:{symbol_upper:0}}])