Мутация AppSync

Я работаю над приложением простых сообщений с AWS AppSync. Это моя схема:

type Conversation {
    id: String!
    messageIds: [String]
}

type ConversationConnection {
    items: [Conversation]
    nextToken: String
}

input CreateConversationInput {
    id: String!
    messageIds: [String]
}

input CreateMessageInput {
    id: String!
    text: String!
    timestamp: String!
}

input CreateProfileInput {
    id: Int!
    name: String!
    profileImage: String
    isOnline: Boolean!
}

input DeleteConversationInput {
    id: String!
}

input DeleteMessageInput {
    id: String!
}

input DeleteProfileInput {
    id: Int!
}

type Message {
    id: String!
    text: String!
    timestamp: String!
}

type MessageConnection {
    items: [Message]
    nextToken: String
}

type Mutation {
    createProfile(input: CreateProfileInput!): Profile
    updateProfile(input: UpdateProfileInput!): Profile
    deleteProfile(input: DeleteProfileInput!): Profile
    createMessage(input: CreateMessageInput!): Message
    updateMessage(input: UpdateMessageInput!): Message
    deleteMessage(input: DeleteMessageInput!): Message
    createConversation(input: CreateConversationInput!): Conversation
    updateConversation(input: UpdateConversationInput!): Conversation
    deleteConversation(input: DeleteConversationInput!): Conversation
    updateConversationMessages(id: String!, messageIds: [String]): Conversation
}

type Profile {
    id: Int!
    name: String!
    profileImage: String
    isOnline: Boolean!
}

type ProfileConnection {
    items: [Profile]
    nextToken: String
}

type Query {
    getProfile(id: Int!): Profile
    listProfiles(first: Int, after: String): ProfileConnection
    getMessage(id: String!): Message
    listMessages(first: Int, after: String): MessageConnection
    getConversation(id: String!): Conversation
    listConversations(first: Int, after: String): ConversationConnection
}

type Subscription {
    onCreateProfile(
        id: Int,
        name: String,
        profileImage: String,
        isOnline: Boolean
    ): Profile
        @aws_subscribe(mutations: ["createProfile"])
    onUpdateProfile(
        id: Int,
        name: String,
        profileImage: String,
        isOnline: Boolean
    ): Profile
        @aws_subscribe(mutations: ["updateProfile"])
    onDeleteProfile(
        id: Int,
        name: String,
        profileImage: String,
        isOnline: Boolean
    ): Profile
        @aws_subscribe(mutations: ["deleteProfile"])
    onCreateMessage(id: String, text: String, timestamp: String): Message
        @aws_subscribe(mutations: ["createMessage"])
    onUpdateMessage(id: String, text: String, timestamp: String): Message
        @aws_subscribe(mutations: ["updateMessage"])
    onDeleteMessage(id: String, text: String, timestamp: String): Message
        @aws_subscribe(mutations: ["deleteMessage"])
    onCreateConversation(id: String, messageIds: [String]): Conversation
        @aws_subscribe(mutations: ["createConversation"])
    onUpdateConversation(id: String, messageIds: [String]): Conversation
        @aws_subscribe(mutations: ["updateConversation"])
    onDeleteConversation(id: String, messageIds: [String]): Conversation
        @aws_subscribe(mutations: ["deleteConversation"])
}

input UpdateConversationInput {
    id: String!
    messageIds: [String]
}

input UpdateMessageInput {
    id: String!
    text: String
    timestamp: String
}

input UpdateProfileInput {
    id: Int!
    name: String
    profileImage: String
    isOnline: Boolean
}

Пока я могу создавать беседы на DynamoDB из клиента, но не обновлять их. Я пытался написать резолвер, но он не работает:

{
    "version": "2017-02-28",
    "operation": "UpdateItem",
    "key": {
        "id": { "S": "${util.context.arguments.id}" }
    }
    "update": {
    "expression": "SET List = :List",
    "expressionValues": {
      #set( $List = $context.arguments.List )
      ":List": $util.dynamodb.toListJson($List)
    }
 }
}

Есть идеи, как добавить идентификатор последнего сообщения в массив messageIds разговора? Спасибо.