Resolve implicit primary key references in table.foreign_keys

For foreign keys declared as "REFERENCES other_table" with no explicit
columns, PRAGMA foreign_key_list returns None for the referenced
column. The foreign_keys property now resolves these to the other
table's primary key columns, so other_column=None unambiguously
indicates a compound foreign key.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

sqlite_utils/db.py

@@ -2075,6 +2075,12 @@ class Table(Queryable):
             other_table = rows[0][1]
             columns = [row[2] for row in rows]
             other_columns = [row[3] for row in rows]
+            if all(c is None for c in other_columns):
+                # "REFERENCES other_table" with no columns - the pragma
+                # returns None, meaning the other table's primary key
+                other_table_pks = self.db.table(other_table).pks
+                if len(other_table_pks) == len(columns):
+                    other_columns = other_table_pks
             is_compound = len(rows) > 1
             fks.append(
                 ForeignKey(

tests/test_foreign_keys.py

@@ -408,3 +408,40 @@ def test_transform_preserves_compound_foreign_key_on_delete():
     assert fk.is_compound is True
     assert fk.on_delete == "CASCADE"
     assert "ON DELETE CASCADE" in db["courses"].schema
+
+
+def test_implicit_primary_key_reference_is_resolved():
+    # REFERENCES authors (no column) has "to" of None in the pragma -
+    # it should be resolved to the primary key of the other table
+    db = Database(memory=True)
+    db.executescript("""
+    CREATE TABLE authors (author_id INTEGER PRIMARY KEY);
+    CREATE TABLE books (
+        id INTEGER PRIMARY KEY,
+        author_id INTEGER REFERENCES authors
+    );
+    """)
+    fk = db["books"].foreign_keys[0]
+    assert fk.is_compound is False
+    assert fk.other_column == "author_id"
+    assert fk.other_columns == ["author_id"]
+
+
+def test_implicit_compound_primary_key_reference_is_resolved():
+    db = Database(memory=True)
+    db.executescript("""
+    CREATE TABLE departments (
+        campus_name TEXT NOT NULL,
+        dept_code TEXT NOT NULL,
+        PRIMARY KEY (campus_name, dept_code)
+    );
+    CREATE TABLE courses (
+        course_code TEXT PRIMARY KEY,
+        campus_name TEXT NOT NULL,
+        dept_code TEXT NOT NULL,
+        FOREIGN KEY (campus_name, dept_code) REFERENCES departments
+    );
+    """)
+    fk = db["courses"].foreign_keys[0]
+    assert fk.is_compound is True
+    assert fk.other_columns == ["campus_name", "dept_code"]