Don't extract NULL values into a lookup row (#186)

sqlite_utils/db.py

@@ -2190,24 +2190,35 @@ def extract(
             )
         lookup_columns = [(rename.get(col) or col) for col in columns]
         lookup_table.create_index(lookup_columns, unique=True, if_not_exists=True)
+        # Don't create a lookup row for the all-NULL combination: a row whose
+        # extracted columns are entirely NULL represents "no value", so it
+        # should keep a NULL foreign key rather than point at a NULL lookup row
+        # (#186). Rows with a partial NULL (some extracted columns set, others
+        # NULL) are a genuine distinct value and are still extracted.
         self.db.execute(
-            "INSERT OR IGNORE INTO {} ({lookup_columns}) SELECT DISTINCT {table_cols} FROM {}".format(
+            "INSERT OR IGNORE INTO {} ({lookup_columns}) SELECT DISTINCT {table_cols} FROM {} WHERE NOT ({all_null})".format(
                 quote_identifier(table),
                 quote_identifier(self.name),
                 lookup_columns=", ".join(quote_identifier(c) for c in lookup_columns),
                 table_cols=", ".join(quote_identifier(c) for c in columns),
+                all_null=" AND ".join(
+                    "{} IS NULL".format(quote_identifier(c)) for c in columns
+                ),
             )
         )
 
         # Now add the new fk_column
         self.add_column(magic_lookup_column, int)
 
-        # And populate it
+        # And populate it. The trailing ``WHERE NOT (<all columns NULL>)`` leaves
+        # all-NULL source rows with a NULL foreign key even when the lookup table
+        # already contains an all-NULL row (e.g. a reused or legacy lookup table)
+        # — the IS-based join would otherwise match that row and link to it (#186).
         self.db.execute(
-            "UPDATE {} SET {} = (SELECT id FROM {} WHERE {where})".format(
-                quote_identifier(self.name),
-                quote_identifier(magic_lookup_column),
-                quote_identifier(table),
+            "UPDATE {table_name} SET {fk} = (SELECT id FROM {lookup} WHERE {where}) WHERE NOT ({all_null})".format(
+                table_name=quote_identifier(self.name),
+                fk=quote_identifier(magic_lookup_column),
+                lookup=quote_identifier(table),
                 where=" AND ".join(
                     "{}.{} IS {}.{}".format(
                         quote_identifier(self.name),
@@ -2217,6 +2228,12 @@ def extract(
                     )
                     for column in columns
                 ),
+                all_null=" AND ".join(
+                    "{}.{} IS NULL".format(
+                        quote_identifier(self.name), quote_identifier(column)
+                    )
+                    for column in columns
+                ),
             )
         )
         # Figure out the right column order

tests/test_extract.py

@@ -177,6 +177,8 @@ def test_extract_error_on_incompatible_existing_lookup_table(fresh_db):
 
 
 def test_extract_works_with_null_values(fresh_db):
+    # A NULL extracted value represents "no value", so it should keep a NULL
+    # foreign key rather than be turned into a lookup row of its own (#186).
     fresh_db["listens"].insert_all(
         [
             {"id": 1, "track_title": "foo", "album_title": "bar"},
@@ -189,9 +191,70 @@ def test_extract_works_with_null_values(fresh_db):
     )
     assert list(fresh_db["listens"].rows) == [
         {"id": 1, "track_title": "foo", "album_id": 1},
-        {"id": 2, "track_title": "baz", "album_id": 2},
+        {"id": 2, "track_title": "baz", "album_id": None},
     ]
     assert list(fresh_db["albums"].rows) == [
         {"id": 1, "album_title": "bar"},
-        {"id": 2, "album_title": None},
+    ]
+
+
+def test_extract_does_not_create_lookup_row_for_all_null(fresh_db):
+    # Single-column: every NULL keeps a NULL fk and no NULL lookup row is made.
+    fresh_db["creatures"].insert_all(
+        [
+            {"id": 1, "name": "Simon", "type": None},
+            {"id": 2, "name": "Natalie", "type": None},
+            {"id": 3, "name": "Cleo", "type": "dog"},
+        ],
+        pk="id",
+    )
+    fresh_db["creatures"].extract("type")
+    assert list(fresh_db["creatures"].rows) == [
+        {"id": 1, "name": "Simon", "type_id": None},
+        {"id": 2, "name": "Natalie", "type_id": None},
+        {"id": 3, "name": "Cleo", "type_id": 1},
+    ]
+    assert list(fresh_db["type"].rows) == [{"id": 1, "type": "dog"}]
+
+
+def test_extract_multi_column_keeps_partial_null_but_not_all_null(fresh_db):
+    # Multi-column: a row whose extracted columns are *all* NULL keeps a NULL
+    # fk, but a partial-NULL combination is a genuine distinct value and is
+    # still extracted (and shared between matching rows) (#186).
+    fresh_db["t"].insert_all(
+        [
+            {"id": 1, "a": "x", "b": None},
+            {"id": 2, "a": "x", "b": None},
+            {"id": 3, "a": None, "b": None},
+            {"id": 4, "a": "y", "b": "z"},
+        ],
+        pk="id",
+    )
+    fresh_db["t"].extract(["a", "b"], table="ab", fk_column="ab_id")
+    rows = list(fresh_db["t"].rows)
+    assert rows[2]["ab_id"] is None  # all-NULL row -> NULL fk
+    assert rows[0]["ab_id"] == rows[1]["ab_id"] is not None  # partial NULL shared
+    assert rows[3]["ab_id"] not in (None, rows[0]["ab_id"])
+    # The lookup table must not contain an all-NULL row.
+    assert not any(row["a"] is None and row["b"] is None for row in fresh_db["ab"].rows)
+
+
+def test_extract_all_null_stays_null_with_preexisting_null_lookup_row(fresh_db):
+    # Reusing a lookup table that already contains an all-NULL row (e.g. created
+    # by an older sqlite-utils version) must still leave all-NULL source rows with
+    # a NULL foreign key — the IS-based join must not link them to that row (#186).
+    fresh_db["type"].insert_all(
+        [{"id": 1, "type": None}, {"id": 2, "type": "dog"}], pk="id"
+    )
+    fresh_db["creatures"].insert_all(
+        [
+            {"id": 1, "name": "Simon", "type": None},
+            {"id": 2, "name": "Cleo", "type": "dog"},
+        ],
+        pk="id",
+    )
+    fresh_db["creatures"].extract("type")
+    assert list(fresh_db["creatures"].rows) == [
+        {"id": 1, "name": "Simon", "type_id": None},
+        {"id": 2, "name": "Cleo", "type_id": 2},
     ]