# Selecting a subset of a table with DRY **URL:** \ **Category:** Rails\ **Created:** [January 10, 2006, 12:08am UTC](https://www.ruby-forum.com/t/selecting-a-subset-of-a-table-with-dry/52001 "2006-01-10T00:08:04Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![vann](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/eada6e/32.png) [@vann](https://www.ruby-forum.com/u/vann)\ **Post date:** [January 10, 2006, 12:08am UTC](https://www.ruby-forum.com/t/selecting-a-subset-of-a-table-with-dry/52001/1 "2006-01-10T00:08:04Z")
Hello, I have table called ‘listings.’ There are a certain combination of properties which make a listing viewable (e.g., it is confirmed, has been authorized, hasn’t expired, etc.). Each listing has one and only one category. I want to be able to do things like Listing.find\_viewable(…) and Category.find(…).viewable\_listings, but I see no way to do this without violating DRY in several places, or stepping all over AR’s toes. My idea was to define a method in application.rb which returns the appropriate SQL string that makes a listing viewable, call it viewable\_sql, and then put that where appropriate. However, I’d like find\_viewable to work just like find, except that it would modify the parameters to find to incorporate those conditions. Any thoughts or examples of something similar? Thanks. – Jesse F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) University of Chicago - NSIT Web Services AIM: farmerje Jabber: removed\_email\_address@domain.invalid Phone: (773)363-1058 ---
**Author:** ![vann](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/eada6e/32.png) [@vann](https://www.ruby-forum.com/u/vann)\ **Post date:** [January 10, 2006, 1:29am UTC](https://www.ruby-forum.com/t/selecting-a-subset-of-a-table-with-dry/52001/2 "2006-01-10T01:29:35Z")
Hello Jesse, 2006/1/9, Jesse F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid): > My idea was to define a method in application.rb which returns the > appropriate SQL string that makes a listing viewable, call it > viewable\_sql, and then put that where appropriate. However, I’d like > find\_viewable to work just like find, except that it would modify the > parameters to find to incorporate those conditions. Did you know that class methods of model objects are made available in has\_many/belongs\_to and friends ? So, if you do class Listing def self.viewables(options={}) options.merge(:conditions =\> [‘confirmed = 1 AND authorized = 1’]) self.find(:all, options) end end Then, you can do Category.listings.viewables(:order =\> ‘created\_at DESC’) Hope that helps ! ---
**Author:** ![vann](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/eada6e/32.png) [@vann](https://www.ruby-forum.com/u/vann)\ **Post date:** [January 10, 2006, 1:56am UTC](https://www.ruby-forum.com/t/selecting-a-subset-of-a-table-with-dry/52001/3 "2006-01-10T01:56:42Z")
Francois B. wrote: > end > > Then, you can do > > Category.listings.viewables(:order =\> ‘created\_at DESC’) > > ## Hope that helps ! > > François Beausoleil > [http://blog.teksol.info/](http://blog.teksol.info/) That is nice. For the like I was defining has\_many :viewable\_listings and then doing Category.find(some\_category).viewable\_listings. This provides another nice perspective. However, this doesn’t entirely solve my problem. It is two-fold, I guess. 1. Sometimes I want to find a specific viewable listing. 2. Sometimes I want to find all viewable listings satisfying certain criteria. The way I have it now is that basically the “is\_viewable?” sort of logic is duplicated for each of these. This will simplify my error handling since the controllers searching for a specific entry will be expecting an exception to be raised if there is an error, while ones requesting multiple entries will expect the returned result to be false. So, for example, I might do this: def view @listing = Listing.find\_viewable(params[:id]) rescue … end while elsewhere I might want def category @listings = Category.find\_by\_name(params[:name]).viewable\_listings unless @listings … end end or some such. Is there any way I can integrate these two things without breaking AR and maintaining DRY? The logic is replicated once in viewable\_listings or equivalent and once in find\_viewable. Thanks again! – Jesse F. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) University of Chicago - NSIT Web Services AIM: farmerje Jabber: removed\_email\_address@domain.invalid Phone: (773)363-1058 ---
**Author:** ![vann](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/eada6e/32.png) [@vann](https://www.ruby-forum.com/u/vann)\ **Post date:** [January 10, 2006, 3:09pm UTC](https://www.ruby-forum.com/t/selecting-a-subset-of-a-table-with-dry/52001/4 "2006-01-10T15:09:03Z")
— quote — Is there any way I can integrate these two things without breaking AR and maintaining DRY? The logic is replicated once in viewable\_listings or equivalent and once in find\_viewable. — quote — so just to make sure i understand this correctly, you want a single method to be able to return either a single viewable listing or multiple viewable listings depending on the argiments? sounds just like what AR::Base.finddoes for you already, for the most part. if you think about it, you’d basically be rewriting AR::Base.find with hardcoded conditions (whatever makes a listing ‘viewable’). all this would do is save you from having to specify your conditions in the find call. if you step back and think about it for a minute, this is the way you would normally do it. def view begin @viewable\_listing = Listing.find(params[:id], :conditions =\> “…”) rescue RecordNotFound # handle record not found exception here end end def category @viewable\_listings = Category.listings.find(:all, :conditions =\> “…”) end with your idea, you want to be able to do: def view begin @viewable\_listing = Listing.find\_viewable(params[:id]) rescue RecordNotFound # handle record not found exception end end def category @viewable\_listings = Category.listings.find\_viewable(:all) end notice the only thing i’ve changed is the method name and removed the :conditions, as the conditions would be handled within your custom find\_viewable method class Listing \< ActiveRecord::Base def self.find\_viewable(args) conditions = “…” # merge in your custom ‘viewable’ conditions into the arguments and then call AR::Base.find self.find(args) end end merging your custom conditions is beyond my abilities at the moment, but i think this is what you are after. you could probably take a look at the source to AR::Base.find to figure it out. hope this helps.