db={"campaigns":[{"_id":1,"type":"FOLLOWUP","keyword":"abc"}],"orders":[{"_id":1,"title":"abc"},{"_id":2,"title":"bca"}]}

db.campaigns.aggregate([{$match:{type:"FOLLOWUP"}},{$lookup:{from:"orders",as:"orders",let:{keyword:"$keyword"},pipeline:[{$match:{$expr:{$regexMatch:{input:"$title",regex:"$$keyword",options:"i"}}}}]}}])