Group authorization in AppSync using IAM authentic

My service requires user groups for authorising access to data.

Group authorization examples in AppSync documentation are based on User Pool claims. I'm using IAM authentication so $context.identity doesn't include claims or any similar information.

See, for example, topic "Use Case: Group Can Create New Record" in: https://docs.aws.amazon.com/appsync/latest/devguide/security-authorization-use-cases.html

#set($expression = "")
#set($expressionValues = {})
#foreach($group in $context.identity.claims.get("cognito:groups"))
    #set( $expression = "${expression} contains(groupsCanAccess, :var$foreach.count )" )
    #set( $val = {})
    #set( $test = $val.put("S", $group))
    #set( $values = $expressionValues.put(":var$foreach.count", $val))
    #if ( $foreach.hasNext )
    #set( $expression = "${expression} OR" )
    #end
#end
{
    "version" : "2017-02-28",
    "operation" : "PutItem",
    "key" : {
        ## If your table's hash key is not named 'id', update it here. **
        "id" : { "S" : "$context.arguments.id" }
        ## If your table has a sort key, add it as an item here. **
    },
    "attributeValues" : {
        ## Add an item for each field you would like to store to Amazon DynamoDB. **
        "title" : { "S" : "${context.arguments.title}" },
        "content": { "S" : "${context.arguments.content}" },
        "owner": {"S": "${context.identity.username}" }
    },
    "condition" : {
        "expression": "attribute_not_exists(id) OR $expression",
        "expressionValues": $utils.toJson($expressionValues)
    }
}

I would expect to just check from User table whether the user is in a group that grants this permission. However, DynamoDB conditions don't seem to support querying other tables.

Today i'm using something similar to your requirement. For that I add a custom header in the amplify request with the JWT token from Cognito User Pool. In my case, I parse the JWT inside a lambda resolver. For your case, you'll need to parse the JWT token in the frontend and send it parsed (and encoded) in the custom header. Inside your resolver you can decode the header value and extract the groups from the claims.