[{"_id":ObjectId("5e456e6b33fef4299aa75a7e"),"title":"Training aaa and bbb","exercises":[{"title":"aaa exercise","goals":["aaa"],"length":10},{"title":"bbb exercise","goals":["bbb"],"length":5}],"createdBy":"dummy"},{"_id":ObjectId("5e456e7f33fef4299aa75a7f"),"title":"Training aaa, ccc","exercises":[{"title":"aaa exercise","goals":["aaa"],"length":5},{"title":"aaa exercise","goals":["aaa"],"length":10},{"title":"ccc exercise","goals":["ccc"],"length":5}],"createdBy":"dummy"},{"_id":ObjectId("5e49b282e0a271e9f57648ff"),"title":"Training aaa 2","exercises":[{"title":"aaa","goals":["aaa"],"length":5},{"title":"ccc exercise","goals":["ccc"],"length":10}],"createdBy":"dummy"}]

db.collection.aggregate([{$match:{"createdBy":"dummy"}},{$unwind:"$exercises"},{$unwind:"$exercises.goals"},{$group:{_id:{trainingId:"$_id",goal:"$exercises.goals",},totalPerTraining:{$sum:"$exercises.length"}}},{$group:{_id:"$_id.goal",lengthMin:{$min:"$totalPerTraining"},lengthMax:{$max:"$totalPerTraining"},lengthAvg:{$avg:"$totalPerTraining"},count:{$sum:1},lengthSum:{$sum:"$exercises.length"}}}])