Get dynamodb items where a nested key exist - node.js

Question

I would like to return items where a nested key exists. I have the following table:
"users": [
{
"active": true,
"apps": {
"app-name-1": {
"active": true,
"group": "aaaaaaaaa",
"settings": {}
}
},
"username: "user1"
},
{
"active": true,
"apps": {
"app-name-2": {
"active": true,
"group": "bbbbbb",
"settings": {}
}
},
"username: "user2"
]
So I want to return all users that have "app-name-1" under "apps". Which operation is the best for this purpose?

Answer 1

The question you need to ask yourself isn't just the "operation", but also how do you model your data in DynamoDB. I.e., how does that JSON array you showed translates into a DynamoDB table, with hash and sort keys?
While DynamoDB nominally does supports nested attributes, this support is actually only partial, with some features (notably secondary indexes) not supporting them, so as I'll show now it is better not to use them. To model your data without nested attributes, what you can do is to use a hash key "username" and sort key "appname". Each item in this table is one app belonging to one user. The user's "active" flag is a bit of a problem in this modeling, but you can implement it by using a fake appname for storing such user parameters.
This modeling makes it efficient to list all applications belonging to one user (I assume you need this feature as well...) but not all users with a certain application. But you were looking for the reverse operation - to get a list of users given an app name.
You can get this reverse with a Scan operation but this is a full-table scan, and accordingly can be very slow and expensive (you'll be paying to read the entire database, even if only part of the data is actually returned to the user).
If efficient search by app is important, you should create a secondary index (GSI) whose hash key is app-name and sort key is user (i.e., the opposite key order from that of the base table). You can then query this index to get - efficiently - the list of usernames that have this app.
Note that such a GSI wouldn't have been possible if you were to insist of modeling your "user" item with nested attributes, because GSIs don't support nested attributes as the key.