Baby timeline 

1.Associate baby to user and show letters associated to the baby on the baby timeline. 
2. Restrict access to babies by current_user
you can use [belongs_to](http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/TableDefinition.html#method-i-belongs_to), but it's an alias for [references](http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/TableDefinition.html#method-i-references), so i'd suggest to use references in the future :)