Get a collection using an array of ids, keeping the order

rubyonrails-talk
1

Say if i have an array of resource ids, of size n. Is there a way to get all the corresponding resources, without doing n individual Find calls?

eg, i could do this:

@resources = array.each{|id| @resources << Resource.find(id)}

But, this requires n Find calls which isn't acceptable.

Any ideas anyone?

thanks max

2

Say if i have an array of resource ids, of size n. Is there a way to get all the corresponding resources, without doing n individual Find calls?

eg, i could do this:

@resources = array.each{|id| @resources << Resource.find(id)}

But, this requires n Find calls which isn't acceptable.

Resource.find @resources

Fred

3

Say if i have an array of resource ids, of size n. Is there a way to get all the corresponding resources, without doing n individual Find calls?

eg, i could do this:

@resources = array.each{|id| @resources << Resource.find(id)}

But, this requires n Find calls which isn't acceptable.

Resource.find @resources

Sorry, only read the body of the message, not the subject! I would sort it after, eg

order_hash = {} array.each_with_index {|id, index| order_hash[id]=index Resource.find(array).sort_by {|r| order_hash[r.id]}

Fred

4

Frederick Cheung wrote:

5

YMMV, but why bother with the hash? Unless you have a truly huge list of id's

@resources = Resource.find(array).sort_by {|r| array.index(r.id)}

You could even hide the implementation:

class Resource    def self.find_by_ordered_ids(*array)      options = {}      options = array.pop if array.last.is_a? Hash      # pass an Array or a list of id arguments      array = array.flatten if array.first.is_a? Array      find(array, options).sort_by {|r| array.index(r.id)}    end end

(Note, untested, but that doesn't mean that it *won't* work :wink:

-Rob

Rob Biedenharn http://agileconsultingllc.com Rob@AgileConsultingLLC.com

6

If you use MySQL, the "field" function allows you to retrieve records in a specific id order:

http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/158dc2d879b2fb1/af78ce75ddfa1ed1

7

Mark Reginald James wrote:

If you use MySQL, the "field" function allows you to retrieve records in a specific id order:

http://groups.google.com/group/rubyonrails-talk/browse_thread/thread/158dc2d879b2fb1/af78ce75ddfa1ed1

-- Rails Wheels - Find Plugins, List & Sell Plugins - http://railswheels.com

Ah, now that is a great tip, i'd not seen this before! Thanks a lot.

The weird thing is, i've gone to my console to try it out, and rails is now always giving me the resources back in the order of the id array anyway, whether i use the field option or not. It's the same with any of my AR classes! I'm sure that this didn't used to happen though...i think i'm going crazy. Can anyone tell me if this is normal? If so then i just imagined this whole problem...

AModel.find([3,1,2]).collect(&:id) => [3,1,2]

or do you get

=> [1,2,3]

?

8

Max Williams wrote:

The weird thing is, i've gone to my console to try it out, and rails is now always giving me the resources back in the order of the id array anyway, whether i use the field option or not. It's the same with any of my AR classes! I'm sure that this didn't used to happen though...i think i'm going crazy. Can anyone tell me if this is normal?

You are not going crazy.

MySQL may, but is not required to, return the objects in the order of the array. So, the behavior will change without warning.

You still need to sort after the fetch.

9

I am! This is a reply to a two-year-old thread... I just wasted 10 minutes trying to find out why I hadn't received the previous messages. I thought my email spam filter was being paranoid :-/