[{"style_id":1,"name":"Forest Green & Black","original_price":"140","sale_price":"0","default?":true,"skus":[{"id":37,"styleId":1,"size":"XS","quantity":16},{"id":38,"styleId":1,"size":"S","quantity":8}]}]

db.collection.aggregate({$addFields:{skus:{$arrayToObject:{$map:{input:"$skus",in:{k:{$toString:"$$this.id"},v:"$$this"}}}}}})