How to update DynamoDB item by searching on GSI primary key? Table has these columns on DynamoDB:


primaryKey
sortKey
GSI_primaryKey
data


And there is GSI added with primary key GSI_primaryKey and sort key on primaryKey

How to update column data if I have only the value of the GSI_primaryKey? 

With NodeJS document client.

Thanks :)
What I found out is I need to get the item by GSI and then using primaryKey and sortKey update the item and cannot update item using GSI only.