compilation fixes

todo.md

@@ -1,5 +1,7 @@
 # Todo
 
+- move `object_definition` and `attribute_definition` to `table` and `column` in query
+  and add `contraint` class
 - replace mk_column with builder style (see `query/builder.hpp`)
 - fix corresponding tests
 - enhance query helper macro to look like the `book` class below
@@ -14,6 +16,7 @@
 - implement lazy loading
 - implement polymorphic class hierarchies
 - finish `schema` and `schema_repository` classes (move add/drop from `session` to `schema`)
+- implement a flag class for enumerations
 
 ## book class
 ```cpp