Database

db={"workouts":[{"_id":{"$oid":"60ffbd531b1cba41e8a7fde3"},"id_user":{"$oid":"60fd50ee1e9d064a5cee0bec"},"date":{"$date":"2021-05-20T22:00:00.000Z"},"name":"wielki bicek","workouts":[{"_id":{"$oid":"60e07e588a2f1946f895475f"},"parts":[{"_id":{"$oid":"60ffbd531b1cba41e8a7fde5"},"weight":"30","repeating":"10"}]},{"_id":{"$oid":"60e07e588a2f1946f895475f"},"parts":[{"_id":{"$oid":"60ffbd531b1cba41e8a7fde9"},"weight":"30","repeating":"10"},{"_id":{"$oid":"60ffbd531b1cba41e8a7fdea"},"weight":"30","repeating":"10"}]}],"__v":0}],"exercises":[{"_id":{"$oid":"60e07e588a2f1946f895475f"},"name":"Wyciskanie sztangi","image":"uploads\\2021-07-03T15-12-24.173Z-test.gif","type":"force","groupMuscle":{"$oid":"60daf099c1aaa20b4cfdf0fa"},"instruction":"Opierając tułów znazewnątrz.","__v":0}]}

Query

db.workouts.aggregate([{"$unwind":"$workouts"},{"$lookup":{"from":"exercises",pipeline:[{"$project":{name:1}}],"localField":"workouts._id","foreignField":"_id","as":"names"}},{"$addFields":{"workouts.i":{"$first":"$names"}}},{"$addFields":{"workouts.name":"$workouts.i.name"}},{"$project":{"names":0,"workouts.i":0}},{"$group":{"_id":{"_id":"$_id","id_user":"$id_user","date":"$date","name":"$name"},"workouts":{$push:"$workouts"}}}])

Result