Updating collection self link for collections page. (#297)

**Related Issue(s):**
- #296

**Description:**
Updating `CollectionLinks` to generate correct `self` link for
`collections` endpoint.

**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

CHANGELOG.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Changed
 
+- Updated CollectionLinks to generate correct `self` link for collections endpoint. [#297](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/297)
 - Refactored aggregation in database logic. [#294](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/294)
 - Fixed the `self` link for the `/collections/{collection_id}/aggregations` endpoint. [#295](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/pull/295)
 

stac_fastapi/core/stac_fastapi/core/models/links.py

@@ -114,6 +114,14 @@ class CollectionLinks(BaseLinks):
     collection_id: str = attr.ib()
     extensions: List[str] = attr.ib(default=attr.Factory(list))
 
+    def link_self(self) -> Dict:
+        """Return the self link."""
+        return dict(
+            rel=Relations.self.value,
+            type=MimeTypes.json.value,
+            href=urljoin(self.base_url, f"collections/{self.collection_id}"),
+        )
+
     def link_parent(self) -> Dict[str, Any]:
         """Create the `parent` link."""
         return dict(rel=Relations.parent, type=MimeTypes.json.value, href=self.base_url)