DynamoDB Friends Query - amazon-web-services

Question

I have DynamoDB table with users and friends.Schema looks like blow. Here user 1 (tom) and user 2 (rob) are friends.
+--------+---------+----------+
| PK | SK | UserName |
+--------+---------+----------+
| USER#1 | USER#1 | tom |
| USER#2 | USER#2 | bob |
| USER#3 | USER#3 | rob |
| FRD#1 | USER#2 | |
| FRD#2 | USER#1 | |
+--------+---------+----------+
Is it possible to get name of friends of user 1 (tom) in single query?
If not what is efficient way to query.
Any help would be really appreciated.
What I am doing currently is:
Step 1: Get all friends of user 1.
let frdParams = {
TableName : "TABLE_NAME",
IndexName: "SK-PK-index",
KeyConditionExpression: "SK = :userId AND begins_with(PK, :friend)",
ExpressionAttributeValues: {
":userId": {S: userId},
":friend": {S: "FRIEND#"}
}
};
const frdRes = await ddb.query(frdParams).promise();
Step 2: Once I getting all users, running more queries in loop.
for (const record of frdRes.Items) {
let recordX = aws.DynamoDB.Converter.unmarshall(record);
let friendId = itemX.PK.replace("FRD", "USER")
let userParams = {
TableName : "TABLE_NAME",
KeyConditionExpression: "PK = :userId AND SK = :userId",
ExpressionAttributeValues: {
":userId": {S: friendId}
}
};
const userRes = await ddb.query(userParams).promise();
}

Answer 1

Data modeling in DynamoDB requires a different mindset than one might use when working with SQL databases. To get the most out of DynamoDB, you need to consider your applications access patterns and store your data in a way that supports those use cases.
It sounds like your access pattern is "fetch friends by user id". There are many ways to implement this access pattern, but I'll give you a few ideas of how it might be achieved.
Idea 1: Denormalize Your Data
You could create a list attribute and store each users friends list. This would make fetching friends by user super simple!
As with any access pattern, there are limitations with this approach. DynamoDB attributes have a maximum size of 400KB, so you'd be limited to a friends list of that size. Also, you will not be able to perform queries based on the values of this attribute, so it would not support additional access patterns. But, it's super simple!
Idea 2: Build an item collection, storing friends within the USER#<id> partition.
This is a typical pattern to represent one-to-many relationships in DynamoDB. Let's say you define friendships with a PK of USER#<user_id> and an SK of FRIEND#<friend_id>. Your table would look like this:
You could fetch the friends of a given user by searching the users partition key for Sort Keys that begins_with FRIEND.
These are just two ideas, and there are many more (and likely better) ways to model friendships in DynamoDB. The examples I've given treat the relationship as one-to-many (one user has many friends). What's more likely is that you'd have a many-to-many relationship to model, which can be tricky in DynamoDB (and another topic altogether!)
If many-to-many sounds like what you have, AWS has an article describing modeling many-to-many relationships that may prove a good starting point.