[{inv_name:"Client 1",inv_date:"",inv_ready:false,inv_payments:[{id:"123",pay_amount:32.45},{id:"456",pay_amount:55.60}]},{inv_name:"Client 2",inv_date:"",inv_ready:true,inv_payments:[{id:"459",pay_amount:67.45},{id:"556",pay_amount:30.60}]}]

db.collection.aggregate([{"$project":{_id:0,"inv_payments":{$map:{input:"$inv_payments",in:{client:"$inv_name",pay_date:"$$this.id",pay_amount:"$$this.pay_amount"}}}}},{$unwind:"$inv_payments"},{$replaceRoot:{newRoot:"$inv_payments"}}])