find_by_sql across multiple tables

rubyonrails-talk
1

Hi,

I have a rather complex algorithm that needs to return an ordered result set, the only problem is that the results contain rows for two different tables, thus meaning if I do Images.find_by_sql, the rows that aren’t Images will be encapsulated by an Image object. The solution I have in mind is to create an ImageIntermediate model, which will return the correct object for a row based on the value inside a column. The problem is that ActiveRecord will think the object returned by ImageIntermediate will be a new record. I can override this with instance_variable_set(“@new_record”, false), but that’s pretty hacky.

Does anyone have a better solution to this problem?

Cheers Ian

2

How complex is the query? select images.* from images…join…join…join will get only images column information.

3

The algorithm calculates the popularity of an asset based on the frequency of votes within a certain time frame, much like digg.com. I have a section for Images which shows images uploaded by users. I’m adding the ability for users to post a link to an image rather than upload one. I want the result set to contain both images and image links. I’ll need to UNION my queries and ORDER them, that’s not a problem… JOINs won’t help as I actually want the correct corresponding model instance out of the other end, hence the need for an ImageIntermediate model that will return either an Image or ImageLink object when I call something like get_obj() on it. I’d just rather avoid the hack that get_obj() will have to perform:

def get_obj

self.is_link? ? obj = ImageLink.new self.attributes : Image.new self.attributes
obj.instance_variable_set("@new_record", false) # hack
obj

end

4

You should be able to use STI here... take a look at http://wiki.rubyonrails.org/rails/pages/SingleTableInheritance

What this would do is let you do

class ImageReference < ActiveRecord::Base end

class Image < ImageReference end

class ImageLink < ImageReference end

Then, in your database, you have one table for the both of them, with a field called 'type'. AR will automatically fill that field with the kind of object it is, Image or ImageLink.

In this case, you'd have a null column in either case (the blob column [or file name column, or whatever you're using for the image] on one side, and the href column for ImageLink), but it still seems as though it would solve your problem.