Rails Ruby on Rails → 2.x

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  Chris O'Sullivan November 21st, 2008 @ 05:25 PM

  (The second patch file for the singular_named_scope method)

  • add_singular_named_scope.patch 2.3 KB

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  Ryan Bates November 21st, 2008 @ 07:25 PM

  Sorry, perhaps I'm missing something. Is there a reason you don't like doing this?

  
  class Book < ActiveRecord::Base
    named_scope :most_recently_read, :order => "last_read desc"
  end
  Book.recently_read.first
  

  To me this is much more useful and flexible. If you need to list multiple recently read books no need to add a second named scope. And this is chainable with other named scopes. If you have a singular named scope you couldn't chain others after it:

  
  Book.recently_read.favorites.first
  Book.most_recently_read.favorites # wouldn't work
  

  Of course you can always chain the singular one at the end, but in general that seems to go against the grain of named scopes to me.

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  Ryan Bates November 21st, 2008 @ 07:26 PM

  Oops, the first code snippet should be:

  
  class Book < ActiveRecord::Base
    named_scope :recently_read, :order => "last_read desc"
  end
  Book.recently_read.first
  

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  Chris O'Sullivan November 23rd, 2008 @ 09:22 PM

  I think it IS in the spirit to want to return one object from a named_scope. After all, .first and .last return only one object (although I guess they're not 'technically' named_scopes.) This patch would allow developers to create named_scopes like oldest or largest - (or if you're so inclined second through forty_second).

  With traditional ActiveRecord finders we don't have to write code like:

  
  Book.find(:all, :order => "last_read desc").first
  

  So why should named_scopes be any different?

  I also prefer the readability of something like:

  Book.last_read

  rather than

  Book.recently_read.first

  And there's no reason I can't have both scopes, one to define the last book that was read, and one to define a list of books ordered by 'most recently read'.

  But, of course there's also nothing to stop me defining a class method like:

  
  def self.last_read
    self.recently_read.first
  end
  

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  Chris Kilmer February 19th, 2009 @ 05:28 PM

  To continue the conversation a little bit. We are currently looking at this exact problem. We want a particular model to return a default. The model can have one and only one model.

  We currently have:

  Device.default.first

  (where Device.default is a named_scope)

  First seems redundant. Default should imply 1.

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  MatthewRudy February 25th, 2009 @ 03:45 PM

  I dont mind having some sort of declaration like this.

  But it isn't a "named_scope" by any means

  
  Book.most_recently_read.count => Exception,
  Book.most_recently_read.in_stock => Exception,
  

  It doesn't scope anything. It doesnt even use the named scope.

  If I were implementing it I'd called it "named_finder"

  
  named_finder :most_recently_read, :first, :order => "last_read desc"
  

  or

  
  named_scope :recently_read, :order => "last_read DESC" do
    named_finder :most_recently_read, :first
  end
  

  It feels like an abuse of the name and the code with the patches submitted so far.

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  MatthewRudy February 25th, 2009 @ 03:52 PM

  
  def named_finder(name, *find_args)
    (class << self; self end).instance_eval do 
      define_method name do
        self.find(*find_args)
      end
    end
  end
  

  and it actually seems to work

  
  >> class User
  >>   named_finder :first_matthew, :first, :conditions => {:firstname => "Matthew"}
  >> end
  => #<Proc:0x03eed6f0@(irb):4>
  
  >> User.first_matthew
  => #<User id: 633, a_whole_load_of_stuff: "redacted", html_email: true>
  

  nice.

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  MatthewRudy February 25th, 2009 @ 03:56 PM

  (I need to have just one lighthouse user)

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  Pratik March 9th, 2009 @ 01:33 PM

  • Assigned user set to “Pratik”
  • Tag changed from “named_scope” to “named_scope, patch”
  • State changed from “new” to “invalid”
  • Title changed from “[PATCH] Add singular version of named_scope that returns first object” to “Add singular version of named_scope that returns first object”

  I don't think named_scopes should be returning a single object. It's scoping, not a finder.

  But please do discuss in the mailing list if you guys think it's worth it.

  Thanks.

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  Timothy Jones February 10th, 2010 @ 05:34 PM

  Anyone care to create a plugin/gem for this, since many of us think it a good idea but it didn't meet Pratik's personal taste?

• 

  Jeremy Kemper February 10th, 2010 @ 06:31 PM

  Let's see a wider body of examples of where you'd use a singular finder+scope.

  Every one I've seen (or thought of) is something like latest: latest version, latest comment, etc. Order + find first.

  Others?

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  Timothy Jones February 10th, 2010 @ 06:49 PM

  Sometimes the result set contains exactly one (or at most one) record, or I just one one random record and really don't care which it is, so order isn't necessary.
  

  For example, in a geographic database containing US cities and states I have a named_scope "Location.city_in_state(city_name, state_abbreviation)". Having to suffix that with ".first" everyplace it's used throughout the application isn't very DRY, is it?

  Basically, named_scope() should have a :first mode for the same reason that find() has a :first mode.

• 

  DHH February 10th, 2010 @ 07:47 PM

  Tim, why not do Location.find_by_city_and_state(city_name, state_abbreviation) in that case?

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  Timothy Jones February 10th, 2010 @ 07:54 PM

  In this case my query is more complicated than the attribute-based finders support AFAIK (specifically it requires a join).

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  Timothy Jones February 10th, 2010 @ 08:05 PM

  Ok, I take that back, now realizing that attribute-based finders do accept general find() options. But the whole point of named_scopes is syntactic sugar, right?

  My Location model is generic, so locations have a name, type, and potentially multiple parents. In the US, cities' parents are counties, then states. In Canada, cities are parented directly by states. Here's my (admittedly gnarly) named_scope:

  named_scope(

  :city_in_state,
  lambda do |c,s|
    {
      :select => "locations.*",
      :joins => "JOIN locations_parents AS l_p ON locations.id = l_p.location_id LEFT JOIN locations AS counties ON l_p.parent_id = counties.id AND counties.location_type_id = #{LocationType.find_by_name('county').id} LEFT JOIN locations_parents AS c_p ON counties.id = c_p.location_id JOIN locations AS states ON (c_p.parent_id = states.id OR l_p.parent_id = states.id) AND states.location_type_id = #{LocationType.find_by_name('state').id}",
      :conditions => ["locations.location_type_id = ? AND locations.name = ? AND (states.name = ? OR states.abbreviation = ?)", LocationType.find_by_name("city").id, c, s, s]
    }
  end
  

  )

  If you can show me how to write that with current finders in a way that's not drastically uglier than:

  city = Location.city_in_state(city_name, state_name)

  then I'll shut up :-).

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  Lachlan Sylvester February 11th, 2010 @ 10:43 AM

  Tim, why not define a scope to find cities within a state, then you can use find_by_name to get the city.

  city = Location.cities_in_state(state_name).find_by_name(city)
  

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  Timothy Jones February 11th, 2010 @ 06:07 PM

  Yes, that would work and is not drastically uglier than what I proposed.

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  Ryan Bigg October 9th, 2010 @ 10:03 PM

  • Tag cleared.
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  Automatic cleanup of spam.

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  bingbing March 26th, 2011 @ 01:41 AM

  omega watch