DynamoDB: Query to find an item in an array of strings - node.js

Question

It's possible that I'm not quite understanding how hash/primary keys work in DynamoDB, but I'm trying to create a model (using Serverless + Dynogels/NodeJS) for a messaging service.
The model looks like this:
const ConversationORM = dynogels.define('Conversation', {
hashKey: 'id',
timestamps: true,
tableName: config.CONVERSATION_TABLE,
schema: {
id: Joi.string(),
users: Joi.array(), // e.g. ['foo', 'bar', 'moo']
messages: Joi.array()
}
})
As you can see, users is an array, which lists the userIds of the conversation's participants.
I need to create a service which finds all conversations that a user is participating in. In MongoDB (which I'm far more familiar with), I'd do something like:
Conversation.find({users: {"$in": ['foo']} }).then(....
Is there something equivalent I can do in DynamoDB? This is an API call that will happen quite often so I'm hoping to make it as efficient as possible.

Answer 1

This answer takes into account a comment on Hunter Frazier's answer saying you don't want to use a Scan.
When using a Query you need specify a single partition key in the operation. In your schema this would mean partitioning on the userid attribute, which is a set. Partition keys in DynamoDB must be a top-level scalar attribute. As userid is not scalar (its a set), you cannot use this attribute as an index, and therefore you cannot do a Query for the conversations a user is part of.
If you need to do this Query, I would suggest revisiting your schema. Specifically I would suggest implementing the Adjacency list pattern which works well in databases containing many-to-many relationships.
You can see some additional notes on the article above I have written on this answer DynamoDB M-M Adjacency List Design Pattern
In your case you would have:
Primary Key: ConversationID
Sort Key: UserID
GSI Primary Key: UserID
You can then use the GSI Primary key in a Query to return all conversations the user is part of.

Answer 2

I'm not familiar with Dynogels or Serverless but if it uses the regular API this might work:
var params = {
ExpressionAttributeNames: {
"U": "users"
},
ExpressionAttributeValues: {
":a": {
S: "John Doe"
}
},
FilterExpression: "Author = :a",
ProjectionExpression: "#U",
TableName: "Conversations"
};
dynamodb.scan(params, function (err, data) {
if (err) console.log(err, err.stack);
else console.log(data);
});