Extract preload_counts in a gem.

.gitignore

@@ -0,0 +1,7 @@
+*.gem
+.bundle
+Gemfile.lock
+pkg/*
+tags
+*.swp
+*.swo

.rvmrc

@@ -0,0 +1 @@
+rvm use 1.8.7@preload_counts

Gemfile

@@ -0,0 +1,4 @@
+source "http://rubygems.org"
+
+# Specify your gem's dependencies in preload_counts.gemspec
+gemspec

Rakefile

@@ -0,0 +1 @@
+require "bundler/gem_tasks"

lib/preload_counts.rb

@@ -0,0 +1,3 @@
+require "preload_counts/version"
+require "preload_counts/ar"
+

lib/preload_counts/ar.rb

@@ -0,0 +1,89 @@
+# This adds a scope to preload the counts of an association in one SQL query. 
+#
+# Consider the following code:
+# Service.all.each{|s| puts s.incidents.acknowledged.count}
+#
+# Each time count is called, a db query is made to fetch the count. 
+#
+# Adding this to the Service class:
+#
+# preload_counts :incidents => [:acknowledged]
+#
+# will add a preload_incident_counts scope to preload the counts and add
+# accessors to the class. So our codes becaumes
+#
+# Service.preload_incident_counts.all.each{|s| puts s.acknowledged_incidents_count}
+#
+# And only requires one DB query.
+module PreloadCounts
+  module ClassMethods
+    def preload_counts(options)
+      options = Array(options).inject({}) {|h, v| h[v] = []; h}  unless options.is_a?(Hash)
+      options.each do |association, scopes|
+        scopes = scopes + [nil]
+
+        # Define singleton metho to load all counts
+        name = "preload_#{association.to_s.singularize}_counts"
+        singleton = class << self; self end
+        singleton.send :define_method, name do
+          sql = ['*'] + scopes_to_select(association, scopes)
+          sql = sql.join(', ')
+          scoped(:select => sql)
+        end
+
+        scopes.each do |scope|
+          # Define accessor for each count
+          accessor_name = find_accessor_name(association, scope)
+          define_method accessor_name do
+            result = send(association)
+            result = result.send(scope) if scope
+            (self[accessor_name] || result.size).to_i
+          end
+        end
+
+      end
+    end
+
+    private
+    def scopes_to_select(association, scopes)
+      scopes.map do |scope|
+        scope_to_select(association, scope)
+      end
+    end
+
+    def scope_to_select(association, scope)
+      resolved_association = association.to_s.singularize.camelize.constantize
+      scope_sql = if scope
+                    resolved_association.scopes[scope].call(resolved_association).send(:construct_finder_sql, {})
+                  else
+                    "1 = 1"
+                  end
+      # FIXME This is a really hacking way of getting the named_scope condition.
+      # In Rails 3 we would have AREL to get to it. 
+      condition = scope_sql.gsub(/^.*WHERE/, '')
+      sql = <<-SQL
+      (SELECT count(*) 
+       FROM #{association} 
+       WHERE #{association}.#{to_s.underscore}_id = #{table_name}.id AND 
+       #{condition}) as #{find_accessor_name(association, scope)} 
+      SQL
+    end
+
+    def find_accessor_name(association, scope)
+      accessor_name = "#{association}_count"
+      accessor_name = "#{scope}_" + accessor_name if scope
+      accessor_name
+    end
+  end
+
+  module InstanceMethods
+  end
+
+  def self.included(receiver)
+    receiver.extend ClassMethods
+    receiver.send :include, InstanceMethods
+  end
+end
+
+ActiveRecord::Base.class_eval { include PreloadCounts }
+

lib/preload_counts/version.rb

@@ -0,0 +1,3 @@
+module PreloadCounts
+  VERSION = "0.0.1"
+end

preload_counts.gemspec

@@ -0,0 +1,24 @@
+# -*- encoding: utf-8 -*-
+$:.push File.expand_path("../lib", __FILE__)
+require "preload_counts/version"
+
+Gem::Specification.new do |s|
+  s.name        = "preload_counts"
+  s.version     = PreloadCounts::VERSION
+  s.authors     = ["Simon Mathieu"]
+  s.email       = ["simon.math@gmail.com"]
+  s.homepage    = ""
+  s.summary     = %q{Preload association or scope counts.}
+  s.description = %q{Preload association or scope counts. This can greatly reduce the number of queries you have to perform and thus yield great performance gains.}
+
+  s.rubyforge_project = "preload_counts"
+
+  s.files         = `git ls-files`.split("\n")
+  s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
+  s.executables   = `git ls-files -- bin/*`.split("\n").map{ |f| File.basename(f) }
+  s.require_paths = ["lib"]
+
+  s.add_development_dependency "rspec"
+  s.add_development_dependency "rails", "~> 2.3.12"
+  s.add_development_dependency "sqlite3"
+end

spec/active_record_spec.rb

@@ -0,0 +1,65 @@
+require 'spec_helper'
+
+ActiveRecord::Base.establish_connection(:adapter => "sqlite3", :dbfile => ":memory:")
+
+def setup_db
+  ActiveRecord::Schema.define(:version => 1) do
+    create_table :posts do |t|
+    end
+
+    create_table :comments do |t|
+      t.integer :post_id, :nulll => false 
+    end
+  end
+end
+
+setup_db 
+
+class Post < ActiveRecord::Base
+  has_many :comments
+  preload_counts :comments => [:with_even_id]
+end
+
+class Comment < ActiveRecord::Base
+  belongs_to :post
+
+  named_scope :with_even_id, lambda { {:conditions => "comments.id % 2 == 0"} }
+end
+
+def create_data
+  post = Post.create 
+  10.times { post.comments.create }
+end
+
+create_data
+
+describe Post do
+  it "should have a preload_comment_counts scope" do
+    Post.should respond_to(:preload_comment_counts) 
+  end
+
+  describe 'instance' do
+    let(:post) { Post.first } 
+
+    it "should have a comment_count accessor" do
+      post.should respond_to(:comments_count) 
+    end
+
+    it "should be able to get count without preloading them" do
+      post.comments_count.should equal(10)
+    end
+  end
+
+  describe 'instance with preloaded count' do
+    let(:post) { Post.preload_comment_counts.first } 
+
+    it "should be able to get the association count" do
+      post.comments_count.should equal(10)
+    end
+
+    it "should be able to get the association count with a named scope" do
+      post.with_even_id_comments_count.should equal(5)
+    end
+  end
+end
+

spec/spec_helper.rb

@@ -0,0 +1,9 @@
+require 'rspec'
+require 'active_record'
+require 'preload_counts'
+
+RSpec.configure do |config|
+  config.color_enabled = true
+  config.formatter     = 'documentation'
+end
+