How do you reference only the persisted records in an active record association
Both the suggestions from @Florent2 and @CDub are sound. However @florent2's suggestion meant hitting the database again (and potentially ditching any preset eager loading which I didn't want to do) and @CDub's suggestion didn't quite work out in terms of code. Here is what I ended up going with:
Returning only persisted records for a particular association
class Magazine < ActiveRecord::Base
has_many :pages do
def persisted
collect{ |page| page if page.persisted? }
end
end
end
this allows you to call .persisted
on any ActiveRecord relation of pages associated with Magazine. It doesn't hit the database again as it simply filters through the pre-loaded objects returning the ones which are persisted.
Making the code reusable
Since I want to reuse this code on a regular basis I can pull it out into a module
module PersistedExtension
def persisted
select{|item| item if item.persisted?}
end
end
It can then be included into the association methods using a lambda:
class Magazine < ActiveRecord::Base
# ...
has_many :pages, -> { extending PersistedExtension }
end
and I can call it intuitively:
@magazine = Magazine.first
@magazine.pages.persisted
# => array of pages which are persisted
# the new persisted association extension works on any AR result set
@magazine.pages.order('page ASC').persisted
Another cleaner syntax, using ActiveRecord where.not
and still get back an ActiveRecord
collection:
- @magazine.pages.where.not(id: nil).each do |page|
...
You could always reject pages that are new records...
%h4 Existing pages
- @magazine.pages.persisted.each do |page|
%p= link_to page, page.title
where on Page
you'd have something like:
def self.persisted
reject {|page| page.new_record? }
end
You can create in your Page model a persisted
scope: scope :persisted, -> { where "id IS NOT NULL" }
, which avoids iterating on each associated page to check whether it's a new record or not.