Add amazon_id column to artists table for full source parity

Schema: ALTER TABLE artists ADD COLUMN amazon_id TEXT with index, added via
_add_amazon_columns migration called after Discogs in _run_migrations.

SOURCE_ID_FIELD: add "amazon" -> "amazon_id" entry. find_library_artist_for_
source now looks up Amazon artists by slug before falling back to name match,
same as every other source. artist_source_detail already stamps artist_info
[source_id_field] = artist_id so the amazon_id is set on source-only payloads.

Tests: add "amazon": "amazon_id" to EXPECTED_SOURCE_ID_FIELD; revert test
assertion back to strict equality (SOURCE_ONLY_ARTIST_SOURCES == SOURCE_ID_
FIELD.keys() holds again now that amazon has a column).

core/artist_source_lookup.py

@@ -36,6 +36,7 @@ SOURCE_ID_FIELD = {
     "discogs": "discogs_id",
     "hydrabase": "soul_id",
     "musicbrainz": "musicbrainz_id",
+    "amazon": "amazon_id",
 }
 
 
@@ -58,18 +59,19 @@ def find_library_artist_for_source(
     Returns ``None`` on miss or on any database error.
     """
     column = SOURCE_ID_FIELD.get(source)
+    if not column:
+        return None
 
     try:
         with database._get_connection() as conn:
             cursor = conn.cursor()
-            if column:
-                cursor.execute(
-                    f"SELECT id, name FROM artists WHERE {column} = ? LIMIT 1",
-                    (str(source_artist_id),),
-                )
-                row = cursor.fetchone()
-                if row:
-                    return row[0]
+            cursor.execute(
+                f"SELECT id, name FROM artists WHERE {column} = ? LIMIT 1",
+                (str(source_artist_id),),
+            )
+            row = cursor.fetchone()
+            if row:
+                return row[0]
 
             if artist_name and active_server:
                 cursor.execute(

database/music_database.py

@@ -407,6 +407,9 @@ class MusicDatabase:
             # Add Discogs enrichment columns (migration)
             self._add_discogs_columns(cursor)
 
+            # Add Amazon artist ID column (migration)
+            self._add_amazon_columns(cursor)
+
             # Backfill match_status for rows that already have an external ID but
             # NULL status. Prevents enrichment workers from re-processing these
             # rows forever. Must run AFTER all *_match_status columns have been
@@ -2035,6 +2038,18 @@ class MusicDatabase:
         except Exception as e:
             logger.error(f"Error adding Discogs columns: {e}")
 
+    def _add_amazon_columns(self, cursor):
+        """Add Amazon artist ID column to artists table."""
+        try:
+            cursor.execute("PRAGMA table_info(artists)")
+            artists_columns = [column[1] for column in cursor.fetchall()]
+            if 'amazon_id' not in artists_columns:
+                cursor.execute("ALTER TABLE artists ADD COLUMN amazon_id TEXT")
+            cursor.execute("CREATE INDEX IF NOT EXISTS idx_artists_amazon_id ON artists (amazon_id)")
+            logger.info("Amazon columns added/verified successfully")
+        except Exception as e:
+            logger.error(f"Error adding Amazon columns: {e}")
+
     def _backfill_match_status_for_existing_ids(self, cursor):
         """Set `<provider>_match_status = 'matched'` for rows that already have a
         populated external ID but NULL match_status.

tests/metadata/test_artist_source_lookup.py

@@ -31,6 +31,7 @@ EXPECTED_SOURCE_ID_FIELD = {
     "discogs": "discogs_id",
     "hydrabase": "soul_id",
     "musicbrainz": "musicbrainz_id",
+    "amazon": "amazon_id",
 }
 
 
@@ -68,11 +69,11 @@ class TestSourceIdFieldMapping:
             "(and the test body) to match."
         )
 
-    def test_source_only_set_contains_all_mapped_sources(self):
-        """Every source with a SOURCE_ID_FIELD column must also be in
-        SOURCE_ONLY_ARTIST_SOURCES. The reverse is not required — sources
-        without a DB column (e.g. amazon) use name-based lookup only."""
-        assert frozenset(SOURCE_ID_FIELD.keys()).issubset(SOURCE_ONLY_ARTIST_SOURCES)
+    def test_source_only_set_matches_mapping_keys(self):
+        """Sources eligible for the source-only fallback must all have a
+        column to look them up by — otherwise the upgrade path silently
+        returns None."""
+        assert SOURCE_ONLY_ARTIST_SOURCES == frozenset(SOURCE_ID_FIELD.keys())
 
     def test_every_mapped_column_exists_on_artists_table(self, db):
         """Regression for the 2026-04 ``deezer_artist_id`` typo: every column