How to query for all posts by followed users in AWS AppSync

Question

I have an AWS Appsync app using DynamoDB as a backend. I have a table for Posts and each post has an username field of the original author. I am in a situation where I want to query to view all post by authors which they are following.

Each User can have many Posts and each User can follow many User.

type Post {
    id: ID!
    username: String
    title: String
    content: String
}

type User{
     username: String
     following: [String]
}

How should I structure the tables in DynamoDB and the AppSync resolvers/schema in AppSync accomplish this.

Answer 1

type User {
    username: String! ## Primary key
    posts: [Post]
    following: [Following]
}
type Following {
    follower: String! ## Primary key
    username: String! ## Sort key
    posts: [Post]
}
type Post {
    username: String! ## Primary key
    postID: ID! 
    title: String
    content: String
}
type Query {
    getPostsByUsername(username: String!): User
}
schema {
    query: Query
}

User.posts resolver:

{
    "version": "2017-02-28",
    "operation": "Query",
    "query": {
        "expression": "username = :username",
        "expressionValues": {
            ":username": $util.dynamodb.toDynamoDBJson($context.source.username)
        }
    }
}

$util.toJson($ctx.result.items)

User.following resolver

{
    "version": "2017-02-28",
    "operation": "Query",
    "query": {
        "expression": "follower = :follower",
        "expressionValues": {
            ":follower": $util.dynamodb.toDynamoDBJson($context.source.username)
        }
    }
}

$util.toJson($ctx.result.items)

Following.posts resolver

{
    "version": "2017-02-28",
    "operation": "Query",
    "query": {
        "expression": "username = :username",
        "expressionValues": {
            ":username": $util.dynamodb.toDynamoDBJson($context.source.username)
        }
    }
}

$util.toJson($ctx.result.items)

Query.getPostsByUsername resolver

{
    "version": "2017-02-28",
    "operation": "GetItem",
    "key": {
        "username": $util.dynamodb.toDynamoDBJson($ctx.args.username),
    }
}

$util.toJson($ctx.result)

Testing

UserTable

PostTable

FollowingTable

Now we can query all posts by username and all friends's posts as well.
Hope it is helpful :)