better support for child objects

[abstratt]

Not all relationships are equal. In aggregation relationships:

• often it makes sense to access child objects only via their parent object
• when creating a child object (say, a comment to a post), it doesn't make sense to let the user to choose the parent to add to, that is implied when the user chose to add the child object

The Posts/comments example breaks those two rules. You can see all comments in a global list, and when adding a comment to a post, the user is required to choose the post they are adding the comment to.

[fzaninotto]

Duplicate of #627 and #308 and #241