added some todo information to clarify insert_query_builder functionality

todo.md

@@ -17,6 +17,18 @@ Order of next steps:
 4. Finish `insert_query_builder`
    - collect all inserts for entities with relations
    - tests
+    - cascade_type: `Insert` 
+    - has_many <-> belongs_to: set root id into the join column of elements
+    - belongs_to <-> has_many: ?
+    - belongs_to <-> has_one: set root id into the foreign object
+    - has_one <-> belongs_to: set root id into the foreign object
+    - has_many_to_many: insert relation table entry
+
+if generator type `manual` => extract pk before insert
+ => 
+if generator type `sequence` or `table` => generate pk before insert
+if generator type `identity` => extract pk after insert and before insert of relation objects
+
 5. Finish `session::insert` method
    - use `insert_query_builder`
    - correct handling of pk generator