db={"transactions":[{"_id":{"$oid":"6480267ab9fe78e82131b737"},"date":{"$date":"2020-06-22T00:00:00.000Z"},"desc":"abcKey1def",},{"_id":{"$oid":"6480267ab9fe78e82131b738"},"date":{"$date":"2020-06-23T00:00:00.000Z"},"desc":"abcdef",}],keyword:[{"_id":{"$oid":"64816f3828372d84a93cd4ad"},"code":123,"desc":"Key1"},{"_id":{"$oid":"648174bf28372d84a93cd4b5"},"code":456,"desc":"Key2",}]}

db.transactions.aggregate([{"$lookup":{"from":"keyword","let":{desc:"$desc"},"pipeline":[{$match:{$expr:{"$regexMatch":{"input":"$$desc","regex":{"$concat":[".*","$desc",".*"]}}}}}],"as":"matchingDocs"}},{"$match":{matchingDocs:[]}}])