db={"characters":[{"_id":"first_astronaut","item":"almonds","price":12,"quantity":2}],"guilds":[{"_id":1,"members":[{"_id":"first_astronaut","rank":1,}]},]}

db.guilds.aggregate([{$lookup:{from:"characters",pipeline:[{$match:{$expr:{$eq:[{$toString:"$members._id"},{$toString:"$$character._id"}]}}}],as:"guild_members"},}])