How to autogenerate global ID using AppSync + DynamoDB - amazon-web-services

Question

How do I generate ID automatically when I use AppSync and DynamoDB as the database source?
I have a type looking like below
type Post {
id: ID!
creator: String!
createdAt: String!
like: Int!
dislike: Int!
frozen: Boolean!
}
and input looking like below
input CreatePostInput {
id: ID!
creator: String!
createdAt: String!
like: Int!
dislike: Int!
frozen: Boolean!
}
and my mutation is obviously the combination of the two
createPost(input: CreatePostInput!): Post
However, when I make inserts I have to do something like below
mutation createPost{
createPost(input:{
id:2
creator:"some creator"
createdAt:"some date"
like:0
dislike:0
frozen:false
}){
id
creator
createdAt
like
dislike
frozen
}
}
There is no way I can insert a post without having to know what the id is up to right now. Is there a way I can provide null or any random id and DynamoDB or AppSync automatically create the next index before inserting into the table?
Error generated when I update the input CreatePostInput to not
accept any ID
{
"data": {
"createPost": null
},
"errors": [
{
"path": [
"createPost"
],
"data": null,
"errorType": "DynamoDB:AmazonDynamoDBException",
"errorInfo": null,
"locations": [
{
"line": 2,
"column": 3,
"sourceName": null
}
],
"message": "One or more parameter values were invalid: Type mismatch for key id expected: S actual: NULL (Service: AmazonDynamoDBv2; Status Code: 400; Error Code: ValidationException; Request ID: 629QEM7MH9BRAJ9MHU3FM1S0U3VV4KQNSO5AEMVJF66Q9ASUAAJG)"
}
]
}
My Resolver Template
{
"version" : "2017-02-28",
"operation" : "PutItem",
"key" : {
## If object "id" should come from GraphQL arguments, change to $util.dynamodb.toDynamoDBJson($ctx.args.id)
"id": $util.dynamodb.toDynamoDBJson($util.autoId()),
},
"attributeValues" : $util.dynamodb.toMapValuesJson($ctx.args)
}

Answer 1

First, update the createPost resolver as:
{
"version": "2017-02-28",
"operation": "PutItem",
"key": {
"id": $util.dynamodb.toDynamoDBJson($util.autoId()),
},
"attributeValues": $util.dynamodb.toMapValuesJson($ctx.args.input),
"condition": {
"expression": "attribute_not_exists(#id)",
"expressionNames": {
"#id": "id",
},
},
}
Then, remove the id field from your CreatePostInput input type:
input CreatePostInput {
creator: String!
createdAt: String!
like: Int!
dislike: Int!
frozen: Boolean!
}
Save both the changes and you should be done.
If you're the one of seeing-is-believing guys like me, take a look at this egghead.io video.

Answer 2

You can auto generate a UUIDv4 using the $util.autoId() velocity helper function. For example, you can write a createPost resolver that uses a server-side generated id like this.
{
"version" : "2017-02-28",
"operation" : "PutItem",
"key" : {
"id": $util.dynamodb.toDynamoDBJson($util.autoId()),
},
"attributeValues" : $util.dynamodb.toMapValuesJson($ctx.args)
}