Access requested fields in resolver mapping template

Question

When we request a GraphQL query, for instance,

query GetPost {
  singlePost(id: 123) {
    id
    title
  }
}

and we have configured a Lambda resolver in AWS AppSync, the request mapping template,

{
    "version": "2017-02-28",
    "operation": "Invoke",
    "payload": {
        "resolve": "singlePost",
        "query": $utils.toJson($context.arguments)
    }
}

allows us to define the event object passed to the lambda handler.

For the above example, our Lambda handler would be invoked with an event event wherein event.payload.query.id == 123 or the like.

According to the docs the $context object comprises,

{
    "arguments" : { ... },
    "source" : { ... },
    "result" : { ... },
    "identity" : { ... },
    "request" : { ... }
}

That said, the documentation does not mention where I can access the requested fields of the GraphQL query.

For the former example, these fields would correspond to ["id", "title"].

In the case that I need to resolve some nested properties, e.g. a tags array, of an object through an expensive operation, e.g. a SQL join, it would be beneficial if I could check if this nested property is actually requested.

This question relates to How to get requested fields inside GraphQL resolver?, however, it differs from in the GraphQL implementation graphql-tools vs AppSync.

Arta

Answer 1

This is actually not something that AppSync supports today, unfortunately. It is, however, a request we've heard from other customers, and I'll use this post as a +1 to prioritize it for a future release.

Goldeneye

Is there any update on this feature? It's something that would GREATLY reduce mapping template bloat in our application.. haven't seen any ongoing conversation about this feature. – Islington

docs.aws.amazon.com/appsync/latest/devguide/… – Egotist

Answer 2

There is now a $context.info.selectionSetList field that includes the requested field names. The official docs provide an example and explain some special use cases around interfaces and aliases.

Missy

Note that when using $utils.toJson() on context.info, the values that selectionSetList return are not serialized by default. See also github.com/aws-amplify/amplify-cli/issues/4869 – Egotist

Answer 3

The document has been updated, AWS added a new field info to the context, you can access the field name as below：

{
    "version" : "2017-02-28",
    "operation" : "Invoke",
    "payload": {
      "resolve": "$ctx.info.fieldName",
      "query": $utils.toJson($context.arguments)
    }
}

You can refer to the document here:

https://docs.aws.amazon.com/appsync/latest/devguide/resolver-context-reference.html#aws-appsync-resolver-context-reference-info

Genethlialogy

info.fieldName is the field on the parent that this resolver is being asked to resolve, not the array of field names being requested on the final object that the original question is asking for. – Hurlow