How to use BlankSlate and Proxy patterns to create a symlink between records?
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21-09-2019 - |
Question
I want to be able to add a record which acts like a symlink. In other words I want to be able to store a record/object which will defer most attributes to another record/object.
The same as on a file system where a symlink points to another file (it own data), yet defers everything else to another file.
class Document < ActiveRecord::Base
end
class Page < Document
end
class Folder < Document
end
class Symlink < ActiveRecord::Base
set_table_name :documents
instance_methods.each { |m| undef_method m if (ActiveRecord::Base.instance_methods(false).include? m) && (!['link', 'link_id', 'link_id='].include? m) }
def method_missing(sym, *args, &block)
puts "Sending #{sym}(#{args.join(',')}) to obj"
link.__send__(sym, *args, &block)
end
def save
raise 'Symlink cant be saved' unless new_record?
super
end
private
def link
@link ||= Document.find(self.link_id)
end
end
At the moment when I try and create a new record I get a stack level too deep for 'link'.
I thought this would be a neat way of solving the problem which could be extracted in to a gem (acts_as_symlink :column => 'parent_id'
).
Another thought was to keep a full copy of the original record and use a after_save callback to update any symlinks or if the symlink is changed, update the original. So not real symlinks more like sync'd copies.
Any thoughts on my code or an alternative?
Solution
Okay I seem to have cracked it, the key was to replace the methods which access the database column with methods which defer to another object. By over-riding the id method I can also determine if save
will save the attributes to either the symlink or the original record. Document
in my case is an acts_as_tree
+ acts_as_list
so I am excluding parent_id
and position
columns from being deferred so I can assign the symlink to a different folder (useless otherwise) and position it within that folder.
class Symlink < Document
attr_accessor :save_to
self.columns.map { |c| c.name }.reject { |c| %w(position parent_id sync_id).include? c }.each do | col |
self.send :define_method, col.to_sym do
source.send(col)
end
end
def id
if save_to == :symlink
return read_attribute(:id)
else
return source.send :id
end
end
def copy?
!sync_id.nil?
end
private
def source
if sync_id
@source ||= Document.find(sync_id)
else
Document.new
end
end
end
Only tested in console so far, but reason why this should not work over the full stack.
If its successful I will think abstracting this out in to a general purpose acts_as_symlink gem.