refactor: only allow one of specific link types TDE-1298 (#1182)

### Motivation

Refactoring to support TDE-1298 - only allow one link of each of the
types `Collection`, `Parent`, `Self`.

### Modifications
Added logic in `ImageryItem`s `add_link` method to recognise when a link
of each type already exists.
Added `__str__` method to `StacMediaType` and `Relation` types, and made
sure the `Link` class will contain strings for `rel` and `type`.

### Verification

`pytest`

scripts/stac/imagery/item.py

@@ -71,4 +71,11 @@ def add_collection(self, collection_id: str) -> None:
         self.add_link(Link(path="./collection.json", rel=Relation.PARENT, media_type=StacMediaType.JSON))
 
     def add_link(self, link: Link) -> None:
-        self.stac["links"].append(link.stac)
+        if self.stac.get("links") and link.stac["rel"] in [
+            Relation.COLLECTION,
+            Relation.PARENT,
+            Relation.SELF,
+        ]:  # STAC specification prescribes there can be only one of these
+            self.stac["links"][:] = [l for l in self.stac["links"] if l.get("rel") != link.stac["rel"]]
+
+        self.stac.setdefault("links", []).append(link.stac)

scripts/stac/link.py

@@ -15,26 +15,29 @@ class Relation(str, Enum):
     DERIVED_FROM = "derived_from"
     """ https://github.com/radiantearth/stac-spec/blob/master/best-practices.md#derived-from-relation-derived_from"""
 
+    def __str__(self) -> str:
+        return self.value
+
 
 # pylint: disable=too-few-public-methods
 class Link:
     """Represents a STAC Link Object (https://github.com/radiantearth/stac-spec/blob/master/commons/links.md#link-object).
 
     Attributes:
         path: A string that represents the actual link in the format of an URL.
-        rel: A string that represents the relationship that the link has to the object it will be added to.
+        rel: `Relation` that represents the relationship that the link has to the object it will be added to.
         media_type: `StacMediaType` of the link file.
         file_content: Optional. The content of the file that will be used to store the checksum in `file:checksum`.
         It assumes using the STAC `file` extension.
     """
 
     stac: dict[str, str]
 
-    def __init__(self, path: str, rel: str, media_type: StacMediaType, file_content: bytes | None = None) -> None:
+    def __init__(self, path: str, rel: Relation, media_type: StacMediaType, file_content: bytes | None = None) -> None:
         self.stac = {
             "href": path,
-            "rel": rel,
-            "type": media_type,
+            "rel": str(rel),
+            "type": str(media_type),
         }
 
         if file_content:

scripts/stac/util/media_type.py

@@ -11,3 +11,6 @@ class StacMediaType(str, Enum):
         
         For STAC Item 
     """
+
+    def __str__(self) -> str:
+        return self.value