Collection-Level Masking Strategy Override and Erase After (#17)

Define two things on Collection Meta to support erasure operations at the row level for database connectors:

- Defines a masking strategy override under Collection Meta that will override Policy-based masking strategies if applicable and supported. 
- Also allow `erase_after` to be specified for DSR's (specifically for database connectors).  We can manually specify that one collection should be erased after another.

CHANGELOG.md

@@ -16,6 +16,11 @@ The types of changes are:
 
 ## [Unreleased](https://github.com/ethyca/fideslang/compare/3.0.5...main)
 
+### Changed
+
+- Add collection-level masking strategy overrides and support specifying one collection to be erased after another [#17](https://github.com/ethyca/fideslang/pull/17)
+
+
 ## [3.0.5](https://github.com/ethyca/fideslang/compare/3.0.4...3.0.5)
 
 ### Added

src/fideslang/models.py

@@ -62,6 +62,18 @@
 )
 
 
+class MaskingStrategies(str, Enum):
+    """Possible options for masking strategy overrides"""
+
+    DELETE = "delete"
+
+
+class MaskingStrategyOverride(BaseModel):
+    """Overrides policy-level masking strategies."""
+
+    strategy: MaskingStrategies
+
+
 class FidesModel(BaseModel):
     """The base model for most top-level Fides objects."""
 
@@ -515,7 +527,9 @@ class CollectionMeta(BaseModel):
     """Collection-level specific annotations used for query traversal"""
 
     after: Optional[List[FidesCollectionKey]] = None
+    erase_after: Optional[List[FidesCollectionKey]] = None
     skip_processing: Optional[bool] = False
+    masking_strategy_override: Optional[MaskingStrategyOverride] = None
 
 
 class DatasetCollection(FidesopsMetaBackwardsCompat):

tests/fideslang/test_validation.py

@@ -15,6 +15,8 @@
     FidesDatasetReference,
     FidesMeta,
     FidesModel,
+    MaskingStrategies,
+    MaskingStrategyOverride,
     Policy,
     PolicyRule,
     PrivacyDeclaration,
@@ -642,7 +644,7 @@ def test_specify_fides_meta_with_custom_request_field(self):
             length=None,
             return_all_elements=None,
             read_only=None,
-            custom_request_field="site_id"
+            custom_request_field="site_id",
         )
 
     def test_specify_both_fidesops_meta_and_fides_meta(self):
@@ -789,6 +791,21 @@ def test_collection_key_has_too_many_components(self):
     def test_valid_collection_key(self):
         CollectionMeta(after=[FidesCollectionKey("test_dataset.test_collection")])
 
+    def test_delete_masking_strategy(self):
+        meta = CollectionMeta(masking_strategy_override={"strategy": "delete"})
+
+        assert meta.masking_strategy_override == MaskingStrategyOverride(
+            strategy=MaskingStrategies.DELETE
+        )
+
+    def test_erase_after(self):
+        meta = CollectionMeta(
+            erase_after=[FidesCollectionKey("test_dataset.test_collection")]
+        )
+
+        assert meta.erase_after == [FidesCollectionKey("test_dataset.test_collection")]
+
+
 
 class TestAnyUrlString:
     def test_valid_url(self):