Disallow all-null meta rows: add CHECK constraint, skip null values on write

- convert_metadata_to_rows returns [] for None values instead of an all-null row
- Remove dead None branch from _scalar_to_row
- Simplify null filter in common.py to just check for row absence
- Add CHECK constraint ck_asset_reference_meta_has_value to model and migration 0003

Amp-Thread-ID: https://ampcode.com/threads/T-019cef4e-5240-7749-bb25-1f17fcf9c09c
Co-authored-by: Amp <amp@ampcode.com>

alembic_db/versions/0003_add_metadata_job_id.py

@@ -48,8 +48,28 @@ def upgrade() -> None:
             "ix_asset_references_preview_id", ["preview_id"]
         )
 
+    # Purge any all-null meta rows before adding the constraint
+    op.execute(
+        "DELETE FROM asset_reference_meta"
+        " WHERE val_str IS NULL AND val_num IS NULL AND val_bool IS NULL AND val_json IS NULL"
+    )
+    with op.batch_alter_table(
+        "asset_reference_meta", naming_convention=NAMING_CONVENTION
+    ) as batch_op:
+        batch_op.create_check_constraint(
+            "ck_asset_reference_meta_has_value",
+            "val_str IS NOT NULL OR val_num IS NOT NULL OR val_bool IS NOT NULL OR val_json IS NOT NULL",
+        )
+
 
 def downgrade() -> None:
+    with op.batch_alter_table(
+        "asset_reference_meta", naming_convention=NAMING_CONVENTION
+    ) as batch_op:
+        batch_op.drop_constraint(
+            "ck_asset_reference_meta_has_value", type_="check"
+        )
+
     with op.batch_alter_table(
         "asset_references", naming_convention=NAMING_CONVENTION
     ) as batch_op:

app/assets/database/models.py

@@ -191,6 +191,10 @@ class AssetReferenceMeta(Base):
         Index("ix_asset_reference_meta_key_val_str", "key", "val_str"),
         Index("ix_asset_reference_meta_key_val_num", "key", "val_num"),
         Index("ix_asset_reference_meta_key_val_bool", "key", "val_bool"),
+        CheckConstraint(
+            "val_str IS NOT NULL OR val_num IS NOT NULL OR val_bool IS NOT NULL OR val_json IS NOT NULL",
+            name="ck_asset_reference_meta_has_value",
+        ),
     )
 
 

app/assets/database/queries/asset_reference.py

@@ -46,15 +46,6 @@ def _check_is_scalar(v):
 
 def _scalar_to_row(key: str, ordinal: int, value) -> dict:
     """Convert a scalar value to a typed projection row."""
-    if value is None:
-        return {
-            "key": key,
-            "ordinal": ordinal,
-            "val_str": None,
-            "val_num": None,
-            "val_bool": None,
-            "val_json": None,
-        }
     if isinstance(value, bool):
         return {"key": key, "ordinal": ordinal, "val_bool": bool(value)}
     if isinstance(value, (int, float, Decimal)):
@@ -68,7 +59,7 @@ def _scalar_to_row(key: str, ordinal: int, value) -> dict:
 def convert_metadata_to_rows(key: str, value) -> list[dict]:
     """Turn a metadata key/value into typed projection rows."""
     if value is None:
-        return [_scalar_to_row(key, 0, None)]
+        return []
 
     if _check_is_scalar(value):
         return [_scalar_to_row(key, 0, value)]

app/assets/database/queries/common.py

@@ -101,20 +101,12 @@ def apply_metadata_filter(
 
     def _exists_clause_for_value(key: str, value) -> sa.sql.ClauseElement:
         if value is None:
-            no_row_for_key = sa.not_(
+            return sa.not_(
                 sa.exists().where(
                     AssetReferenceMeta.asset_reference_id == AssetReference.id,
                     AssetReferenceMeta.key == key,
                 )
             )
-            null_row = _exists_for_pred(
-                key,
-                AssetReferenceMeta.val_json.is_(None),
-                AssetReferenceMeta.val_str.is_(None),
-                AssetReferenceMeta.val_num.is_(None),
-                AssetReferenceMeta.val_bool.is_(None),
-            )
-            return sa.or_(no_row_for_key, null_row)
 
         if isinstance(value, bool):
             return _exists_for_pred(key, AssetReferenceMeta.val_bool == bool(value))