When will ActiveRecord save associations?

Unfortunately there are no such thing like associations_to_save. However there are some rules saying what is being saved when. You can find those here: http://guides.rubyonrails.org/association_basics.html. Points: 4.1.5 (belongs_to), 4.2.5 (has_one), 4.3.4 (has_many) and 4.4.4 (habtm).

UPDATE:

In case of has_many association, the child is saved on saving the parent if child.new_record? returns true (child was not yet saved to db), or the foreign_key column needs to be updated. This is why:

1. Adding object to association on saved parent do save new child.
2. Adding object to association on unsaved parent doesn’t save (no foreign key value)
3. If unsaved parent is being saved and has some child objects in association cache, those objects are saved to update foreign_key.