Adding nested fields with type `auto` causes document indexing errors

Description

I've encountered an issue when adding nested fields of type auto. Creating all the nested type auto fields when the collection is created works, but creating one in a separate operation seems to cause field type errors. I'm not sure if I'm missing something or if this is a bug.

Steps to reproduce

1. Create a collection with a nested field of type auto e.g:

   curl "https://****.typesense.net/collections" \
     -X POST \
     -H "Content-Type: application/json" \
     -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}" \
     -d '{
           "enable_nested_fields": true,
           "fields": [
             {
               "name": "name",
               "type": "string"
             },
             {
               "facet": true,
               "name": "nested.foo",
               "optional": true,
               "type": "auto"
             }
           ],
           "name": "test_collection"
         }'
   

   Response:

   {
     "created_at": 1724205875,
     "default_sorting_field": "",
     "enable_nested_fields": true,
     "fields": [
       {
         "facet": false,
         "index": true,
         "infix": false,
         "locale": "",
         "name": "name",
         "optional": false,
         "sort": false,
         "stem": false,
         "store": true,
         "type": "string"
       },
       {
         "facet": true,
         "index": true,
         "infix": false,
         "locale": "",
         "name": "nested.foo",
         "optional": true,
         "sort": false,
         "stem": false,
         "store": true,
         "type": "auto"
       }
     ],
     "name": "test_collection",
     "num_documents": 0,
     "symbols_to_index": [],
     "token_separators": []
   }
   

2. Upload a document that adheres to the schema:

   curl "https://****.typesense.net/collections/test_collection/documents" \
     -X POST \
     -H "Content-Type: application/json" \
     -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}" \
     -d '{
           "name": "mydoc",
           "nested": {
             "foo": "hello"
           }
         }'
   

   This is successfully inserted:

   {
     "id": "0",
     "name": "mydoc",
     "nested": {
       "foo": "hello"
     }
   }
   

3. Update the collection with another "sibling" nested field of type auto:

   curl "https://****.typesense.net/collections/test_collection" \
     -X PATCH \
     -H "Content-Type: application/json" \
     -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}" \
     -d '{
           "fields": [
             {
               "name": "nested.bar",
               "type": "auto",
               "optional": true,
               "facet": true
             }
           ]
         }'
   

   Response:

   {
     "fields": [
       {
         "facet": true,
         "name": "nested.bar",
         "optional": true,
         "type": "auto"
       }
     ]
   }
   

4. Upload another document that adheres to the new schema:

   curl "https://****.typesense.net/collections/test_collection/documents" \
     -X POST \
     -H "Content-Type: application/json" \
     -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}" \
     -d '{
           "name": "indeedadoc",
           "nested": {
             "foo": "hello",
             "bar": "world"
           }
         }'
   

   The document fails to index with an error about nested.bar's type:

   {
     "message": "Field `nested.bar` has an incorrect type."
   }
   

---

This does work as expected if I create both nested fields at once, however:

1. Create the collection with both nested fields in the schema:

   curl "https://****.typesense.net/collections" \
     -X POST \
     -H "Content-Type: application/json" \
     -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}" \
     -d '{
           "enable_nested_fields": true,
           "fields": [
             {
               "name": "name",
               "type": "string"
             },
             {
               "facet": true,
               "name": "nested.foo",
               "optional": true,
               "type": "auto"
             },
             {
               "facet": true,
               "name": "nested.bar",
               "optional": true,
               "type": "auto"
             }
           ],
           "name": "test_collection2"
         }'
   

   Result:

   {
     "created_at": 1724205997,
     "default_sorting_field": "",
     "enable_nested_fields": true,
     "fields": [
       {
         "facet": false,
         "index": true,
         "infix": false,
         "locale": "",
         "name": "name",
         "optional": false,
         "sort": false,
         "stem": false,
         "store": true,
         "type": "string"
       },
       {
         "facet": true,
         "index": true,
         "infix": false,
         "locale": "",
         "name": "nested.foo",
         "optional": true,
         "sort": false,
         "stem": false,
         "store": true,
         "type": "auto"
       },
       {
         "facet": true,
         "index": true,
         "infix": false,
         "locale": "",
         "name": "nested.bar",
         "optional": true,
         "sort": false,
         "stem": false,
         "store": true,
         "type": "auto"
       }
     ],
     "name": "test_collection2",
     "num_documents": 0,
     "symbols_to_index": [],
     "token_separators": []
   }
   

2. Insert a document that conforms to the schema:

   curl "https://****.typesense.net/collections/test_collection2/documents" \
     -X POST \
     -H "Content-Type: application/json" \
     -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}" \
     -d '{
           "name": "indeedadoc",
           "nested": {
             "foo": "hello",
             "bar": "world"
           }
         }'
   

   The document inserts successfully:

   {
     "id": "0",
     "name": "indeedadoc",
     "nested": { "bar": "world", "foo": "hello" }
   }
   

---

The two collection schemas are identical when fetched, other than name and created_at:

curl "https://****.typesense.net/collections/test_collection" \
  -X GET \
  -H "Content-Type: application/json" \
  -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}"

Result:

{
  "created_at": 1724205875,
  "default_sorting_field": "",
  "enable_nested_fields": true,
  "fields": [
    {
      "facet": false,
      "index": true,
      "infix": false,
      "locale": "",
      "name": "name",
      "optional": false,
      "sort": false,
      "stem": false,
      "store": true,
      "type": "string"
    },
    {
      "facet": true,
      "index": true,
      "infix": false,
      "locale": "",
      "name": "nested.foo",
      "optional": true,
      "sort": false,
      "stem": false,
      "store": true,
      "type": "auto"
    },
    {
      "facet": true,
      "index": true,
      "infix": false,
      "locale": "",
      "name": "nested.bar",
      "optional": true,
      "sort": false,
      "stem": false,
      "store": true,
      "type": "auto"
    }
  ],
  "name": "test_collection",
  "num_documents": 1,
  "symbols_to_index": [],
  "token_separators": []
}

curl "https://****.typesense.net/collections/test_collection2" \
  -X GET \
  -H "Content-Type: application/json" \
  -H "X-TYPESENSE-API-KEY: ${TYPESENSE_API_KEY}"

Result:

{
  "created_at": 1724205997,
  "default_sorting_field": "",
  "enable_nested_fields": true,
  "fields": [
    {
      "facet": false,
      "index": true,
      "infix": false,
      "locale": "",
      "name": "name",
      "optional": false,
      "sort": false,
      "stem": false,
      "store": true,
      "type": "string"
    },
    {
      "facet": true,
      "index": true,
      "infix": false,
      "locale": "",
      "name": "nested.foo",
      "optional": true,
      "sort": false,
      "stem": false,
      "store": true,
      "type": "auto"
    },
    {
      "facet": true,
      "index": true,
      "infix": false,
      "locale": "",
      "name": "nested.bar",
      "optional": true,
      "sort": false,
      "stem": false,
      "store": true,
      "type": "auto"
    }
  ],
  "name": "test_collection2",
  "num_documents": 1,
  "symbols_to_index": [],
  "token_separators": []
}

Expected Behavior

Indexing a document should work the same way when a nested field with type auto is either:

• Present when the collection is initially created
• Added after the collection is created

Actual Behavior

Adding a document fails when an nested auto field is added after a collection is created.

Metadata

Typesense Version: v27.0.rc34

OS: Typesense Cloud

Run the curl reproduction in the issue, comparing a nested auto field declared at collection creation with one added through the collection PATCH endpoint. Trace the nested-field schema and document-indexing paths for the mismatch. Done means documents containing the newly added nested auto field index successfully, matching the behavior of fields declared initially.