Assigning an object to a belongs_to association does not automatically save the object. It does not save the associated object either.
When you assign an object to a has_one association, that object is automatically saved (in order to update its foreign key).
In addition, any object being replaced is also automatically saved, because its foreign key will change too
If either of these saves fails due to validation errors, then the assignment statement returns false and the assignment itself is cancelled.
If the parent object (the one declaring the has_one association) is unsaved (that is, new_record? returns true) then the child objects are not saved. They will automatically when the parent object is saved.