Solr Nested Document w/ Field Labelled Relationship: Single Elements Not Stored In Arrays

While indexing my data, I found that some nested documents are not stored correctly. I run Solr 8.3 and make use of the labelled relations as described in the docs.

Data

Whenever a root entity, Parent, has any amount of Child entities, I generate the following PHP array:

[
  'id' => 'b14ac9a0-e255-468b-a673-e125fd73d6f2',
  'entity_type' => 'parent',
  'title_t' => 'Andrea Cook',
  'children' => [
    0 => [
      'id' => 'ce10380c-8006-4945-9078-296116ad5ab7',
      'entity_type' => 'child',
      'title_t' => 'Jordan Gibson',
    ],
    1 => [
      'id' => '0c191119-fae9-452e-aca2-b724a381f939',
      'entity_type' => 'child',
      'title_t' => 'Jane Gordon',
    ],
  ]
]

This is then encoded to the following JSON object:

{
  "id": "b14ac9a0-e255-468b-a673-e125fd73d6f2",
  "entity_type": "parent",
  "title_t": "Andrea Cook",
  "children": [
    {
      "id": "ce10380c-8006-4945-9078-296116ad5ab7",
      "entity_type": "child",
      "title_t": "Jordan Gibson"
    },
    {
      "id": "0c191119-fae9-452e-aca2-b724a381f939",
      "entity_type": "child",
      "title_t": "Jane Gordon"
    }
  ]
}

Which is in turn exactly what is returned by solr queries (plus the auto-generated values __root__, __nest_path__, etc).

Problem

Whenever a Parent only has one Child, they end up as an object in solr, instead of an array containing a single object.

Expected Seach Result

{
  "id": "b14ac9a0-e255-468b-a673-e125fd73d6f2",
  "entity_type": "parent",
  "title_t": "Andrea Cook",
  "children": [
    {
      "id": "ce10380c-8006-4945-9078-296116ad5ab7",
      "entity_type": "child",
      "title_t": "Jordan Gibson"
    }
  ]
}

Real Search Result

{
  "id": "b14ac9a0-e255-468b-a673-e125fd73d6f2",
  "entity_type": "parent",
  "title_t": "Andrea Cook",
  "children": {
    "id": "ce10380c-8006-4945-9078-296116ad5ab7",
    "entity_type": "child",
    "title_t": "Jordan Gibson"
  }
}

Assertions

I have made sure that the php array and JSON object are correctly formed until the moment they are passed to the HTTP client.

I have made sure that the array keys for children arrays are numbered and start at 0.

Questions

Is that the expected behaviour?

Do I have to create a <fieldType/> for the labeled relations (i.e. create a multivalued children field)? If yes, then how would I do that? I haven't found any explanation yet.

What can I do to always get an array for children in my search results, so that I don't have to check the data before iterating over it?

uuidgenerator.net, uinames.com

Solution

It finally occurred to me, that the array manipulation I programmed in PHP (array_map, array_merge, ...) were creating arrays with weird indices.

In order to fix that, I had to call array_values, which creates pristine, zero-indexed arrays.