Enabling asset indexing (stac-utils#341)

**Description:**

There are a number of extensions that include extra fields within an
Item's assets. It would be useful to allow users to filter items based
on these fields.

Mapping assets from an Object to a Array of Objects prevents "mapping
explosion" from asset keys without needing to disable indexing on the
assets.

It does have some search implications. See first "note"
[here](https://www.elastic.co/guide/en/elasticsearch/reference/current/array.html).

**PR Checklist:**

- [x] Code is formatted and linted (run `pre-commit run --all-files`)
- [x] Tests pass (run `make test`)
- [x] Documentation has been updated to reflect changes, if applicable
- [x] Changes are added to the changelog

Author: rhysrevans3

CHANGELOG.md

@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Changed
 
+- Changed assets serialization to prevent mapping explosion while allowing asset inforamtion to be indexed. [#341](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/341)
+
 ### Fixed
 
 ## [v6.2.1] - 2025-09-02

stac_fastapi/core/stac_fastapi/core/serializers.py

@@ -66,6 +66,10 @@ def stac_to_db(cls, stac_data: stac_types.Item, base_url: str) -> stac_types.Ite
         item_links = resolve_links(stac_data.get("links", []), base_url)
         stac_data["links"] = item_links
 
+        stac_data["assets"] = [
+            {"es_key": k, **v} for k, v in stac_data.get("assets", {}).items()
+        ]
+
         now = now_to_rfc3339_str()
         if "created" not in stac_data["properties"]:
             stac_data["properties"]["created"] = now
@@ -103,7 +107,7 @@ def db_to_stac(cls, item: dict, base_url: str) -> stac_types.Item:
             bbox=item.get("bbox", []),
             properties=item.get("properties", {}),
             links=item_links,
-            assets=item.get("assets", {}),
+            assets={a.pop("es_key"): a for a in item.get("assets", [])},
         )
 
 
@@ -128,6 +132,9 @@ def stac_to_db(
         collection["links"] = resolve_links(
             collection.get("links", []), str(request.base_url)
         )
+        collection["assets"] = [
+            {"es_key": k, **v} for k, v in collection.get("assets", {}).items()
+        ]
         return collection
 
     @classmethod
@@ -174,5 +181,9 @@ def db_to_stac(
             collection_links += resolve_links(original_links, str(request.base_url))
         collection["links"] = collection_links
 
+        collection["assets"] = {
+            a.pop("es_key"): a for a in collection.get("assets", [])
+        }
+
         # Return the stac_types.Collection object
         return stac_types.Collection(**collection)

stac_fastapi/sfeos_helpers/stac_fastapi/sfeos_helpers/mappings.py

@@ -134,7 +134,7 @@ class Geometry(Protocol):  # noqa
         "id": {"type": "keyword"},
         "collection": {"type": "keyword"},
         "geometry": {"type": "geo_shape"},
-        "assets": {"type": "object", "enabled": False},
+        "assets": {"type": "object"},
         "links": {"type": "object", "enabled": False},
         "properties": {
             "type": "object",