How can we get nested Avro schema field inside mapping.groovy

Question

here below is my avro schema which contains ext as a nested json

{
  "name": "MyClass",
  "type": "record",
  "namespace": "com.acme.avro",
  "fields": [
    {
      "name": "channel",
      "type": "string"
    },
    {
      "name": "eventType",
      "type": "string"
    },
    {
      "name": "language",
      "type": "string"
    },
    {
      "name": "currency",
      "type": "string"
    },
    {
      "name": "page",
      "type": "string"
    },
    {
      "name": "pos",
      "type": "string"
    },
    {
      "name": "browser_id",
      "type": "string"
    },
    {
      "name": "session_id",
      "type": "string"
    },
    {
      "name": "customer_id",
      "type": "string"
    },
    {
      "name": "created_at",
      "type": "string"
    },
    {
      "name": "ext",
      "type": {
        "name": "ext",
        "type": "record",
        "fields": [
          {
            "name": "product_name",
            "type": "string"
          }
        ]
      }
    },
    {
      "name": "purchase___reference_id",
      "type": "string"
    },
    {
      "name": "purchase___status",
      "type": "string"
    }
  ]
}

and i am trying to get it into my mapping.groovy file

mapping {

  map eventParameters().value('channel') onto 'channel'
  map eventParameters().value('eventType') onto 'eventType'
  map sessionId() onto 'session_id'
  map eventParameters().value('customerId') onto 'customer_id'
  map eventParameters().value('currency') onto 'currency'
  map eventParameters().value('page') onto 'page'
  map eventParameters().value('pos') onto 'pos'
  map eventParameters().value('browser_id') onto 'browser_id'
  /* map timestamp() onto 'created_at' */
  
  map eventParameters().value('ext.product_name') onto 'ext.product_name'

  map eventParameters().value('reference_id') onto 'purchase___reference_id'
  map eventParameters().value('status') onto 'purchase___status'

  
}

but im facing this error

Exception in thread "main" io.divolte.server.recordmapping.SchemaMappingException: Field ext.product_name does not exist in Avro schema; error in mapping eventParameters().value(ext.product_name) onto ext.product_name