Make any order chainable

Question

I want to sort my Person model objects based on a complicated criterion that can't be summarized in a single query - and so in a named scope. Right now I use a class method like:

def Person.very_complicated_sorting
  Person.all.sort { |x,y| x.bunch_of_calculations <=> y.bunch_of_calculations }
end

Is there a way to make this chainable? E.g.

Person.tallest.very_complicate_sorting.youngest

where tallest and youngest are two named scopes.

Answer 1

This isn't possible unfortunately.

The way named scopes work is by "lazily" building up a combined set of SQL parameters, which aren't evaluated until you actually try to do something with them. So for example the following chain of named scopes:

people = Person.tallest.youngest

will not cause any database query to be run, it actually causes an ActiveRecord::NamedScope object to be stored in the people variable. Only when you access or iterate through that object is the SQL run and the objects loaded.

Your problem is that your sorting method isn't being expressed in SQL, it's a set of Ruby conditions. When Rails gets to your sort_by it has to go and fetch and instantiate the Person objects so that it can run your condition on them. After it's done that you have an Array of objects and not a NamedScope object any more.

Answer 2

Your very_complicated_sorting should be in your Person model.

You musn't not write Person.all inside the method, it couldn't be chainable otherwise.

Just keep:

def self.very_complicated_sorting
  sort_by { |x,y| x.bunch_of_calcultaions <=> y.bunch_of_calculations }
end

Answer 3

This is possible if you use Sequel instead of ActiveRecord for your database library. It lets you use def_dataset_method on Models to create methods that operate on Datasets (of which Models are included); as long as your methods return a Dataset then you can chain them as you like. For example:

class Person < Sequel::Model
  def_dataset_method :active_only do
    filter :active=>true
  end
  def_dataset_method :sort_much do
    order :name, :age.desc
  end
end

active_peeps = Person.active_only.sort_much
peeps_active = Person.sort_much.active_only

To stay in Dataset land requires you to only use methods that can be expressed as SQL. You cannot expect to ask for some records from the database, then perform complex Ruby-only logic on them (say, sorting them by their object_id looked up in a Ruby Hash), and then continue to perform SQL on the resulting array. Ruby does not run on your database (other than pl/Ruby).

Alternatively, you can go straight to a Ruby array and then chain whatever Ruby methods you like in any order, if they all operate on arrays and return arrays. You must make a clear decision in your code as to when you are going to fetch the results from the Database and then operate on the array afterwards.