Comparing lists of field-hashes with equivalent AR-objects.
- by Tim Snowhite
I have a list of hashes, as such:
incoming_links = [
{:title => 'blah1', :url => "http://blah.com/post/1"},
{:title => 'blah2', :url => "http://blah.com/post/2"},
{:title => 'blah3', :url => "http://blah.com/post/3"}]
And an ActiveRecord model which has fields in the database with some matching rows, say:
Link.all =>
[<Link#2 @title='blah2' @url='...post/2'>,
<Link#3 @title='blah3' @url='...post/3'>,
<Link#4 @title='blah4' @url='...post/4'>]
I'd like to do set operations on Link.all with incoming_links so that I can figure out that <Link#4 ...> is not in the set of incoming_links, and {:title => 'blah1', :url =>'http://blah.com/post/1'} is not in the Link.all set, like so:
#pseudocode
#incoming_links = as above
links = Link.all
expired_links = links - incoming_links
missing_links = incoming_links - links
expired_links.destroy
missing_links.each{|link| Link.create(link)}
One route I've tried:
I'd rather not rewrite Array#- and such, and I'm okay with converting incoming_links to a set of unsaved Link objects; so I've tried overwriting hash eql? and so on in Link so that it ignored the id equality that AR::Base provides by default. But this is the only place this sort of equality should be considered in the application - in other places the Link#id default identity is required. Is there some way I could subclass Link and apply the hash, eql?, etc overwriting there?
The other route I've tried is to pull out the attributes hash for each Link and doing a .slice('id',...etc) to prune the hashes down. But this requires writing seperate methods for keeping track of the Link objects while doing set operations on the hashes, or writing seperate Collection classes to wrap the incoming_links hash-list and Link-list which seems a bit overkill.
What is the best way to design this interaction? Extra credit for cleanliness.
