Deserialize AWS Result / Nested JSON with JMS Serializer

Question

I'm trying to deserialize data from AWS CloudSearch with JMS Serializer.

The JSON-encoded data from AWS Result looks something like this:

{
  id: 1234, 
  fields: { 
    "field1": ["value1"], 
    "field2": ["value2"] 
  }
}

The entity serializer config:

AppBundle\Entity\MyObject:
  properties:
    id:
      type:   integer
    field1:
      type:   string
      serialized_name: fields['field1']
      // also tried: fields.field1, fields{field1}, and other permutations

The resulting object:

object(AppBundle\Entity\MyObject)
  private 'id' => int 1234
  private 'field1' => null

Is there some way to specify a hash key to use for serialized_name or do I need to preprocess/flatten the data before deserializing?