Children

A parent doesn't declare its children. The relationship is stated once, on the child's reference. A query that wants a project's todos names that field, here in an include::

include: {    todos: Todo.project_id}

It gets a list, since project_id isn't unique, or at most one record when the field is unique. How a query includes records is Includes.

A many-to-many link, such as a project's members, is an entity with two references and a unique rule over both.