Deal with null foreign keys

graph.py

@@ -126,12 +126,14 @@ def _add_related_rows_to_graph(row, row_node, graph):
         except TypeError:
             # This path for foreign keys.
             related_row = related_rows
-            related_node = Node(
-                table=_get_table_name_from_row(related_row),
-                primary_key=_get_primary_key_from_row(related_row),
-                data=_get_data(related_row),
-            )
-            related.append((related_row, related_node))
+            # Ignore null foreign-keys.
+            if related_row is not None:
+                related_node = Node(
+                    table=_get_table_name_from_row(related_row),
+                    primary_key=_get_primary_key_from_row(related_row),
+                    data=_get_data(related_row),
+                )
+                related.append((related_row, related_node))
     unvisited = [
         (row, node) for (row, node) in related
         if node not in graph.nodes

pyproject.toml

@@ -3,7 +3,7 @@ requires = ["hatchling"]
 build-backend = "hatchling.build"
 [project]
 name = "fk-graph"
-version = "0.0.4"
+version = "0.0.5"
 authors = [
   { name="Andrew Curtis", email="[email protected]" },
   { name="John C Thomas" },

test_graph.py

@@ -151,6 +151,14 @@ def test_can_restrict_to_selected_tables(self):
                 any([n.table == "table_c" for n in graph.nodes])
             )
 
+    def test_can_create_graph_when_some_rows_have_null_foreign_keys(self):
+        self._create_entries_with_null_foreign_key(self.engine)
+
+        graph = get_graph(self.engine, "table_a", "1")
+
+        # Check does not add null nodes to graph.
+        self.assertEqual(len(graph.nodes), 1)
+
     def _create_single_table_no_relations(self, engine):
         metadata_object = MetaData()
         table_a = Table(
@@ -312,6 +320,29 @@ def _create_data_with_non_primary_key_non_foreign_kay_fields(self, engine):
             )
             conn.commit()
 
+    def _create_entries_with_null_foreign_key(self, engine):
+        metadata_object = MetaData()
+        table_a = Table(
+            "table_a",
+            metadata_object,
+            Column("id", Integer, primary_key=True),
+            Column("b_id", ForeignKey("table_b.id"), nullable=True),
+        )
+        table_b = Table(
+            "table_b",
+            metadata_object,
+            Column("id", Integer, primary_key=True),
+        )
+        metadata_object.create_all(engine)
+        with engine.connect() as conn:
+            conn.execute(
+                insert(table_a),
+                [
+                    {"id": 1, "b_id": None},
+                ]
+            )
+            conn.commit()
+
 
 def _to_table_primary_keys(nodes):
     return [