mongodb - mobngoDB: set sequential value by group -

i have collection documents have index property indicating order , group property:

{     "item": "a1",     "group": "a",     "index": 1 } {     "item": "a2",     "group": "a",     "index": 2 } {     "item": "b1",     "group": "b",     "index": 3 } {     "item": "b2",     "group": "b",     "index": 4 } {     "item": "c1",     "group": "c",     "index": 5 }  {     "item": "c2",     "group": "c",     "index": 6 }  

now want assign new index values based on ordering of groups: e.g. [b,c,a] should result in:

{     "item": "b1",     "group": "b",     "index": 1 } {     "item": "b2",     "group": "b",     "index": 2 } {     "item": "c1",     "group": "c",     "index": 3 }  {     "item": "c2",     "group": "c",     "index": 4 }  {     "item": "a1",     "group": "a",     "index": 5 } {     "item": "a2",     "group": "a",     "index": 6 } 

is there way in 1 update statement or have run query each document seperately? important initial ordering within group not changed.