Love named_scope

Bad Smell

class PostsController < ApplicationController
  def search
    conditions = { :title => "%#{params[:title]}%" } if params[:title]
    conditions.merge! { :content => "%#{params[:content]}%" } if params[:content]

    case params[:order]
    when "title" : order = "title desc"
    when "created_at : order = "created_at desc"
    end

    @posts = Post.find(:all, :conditions => conditions, :order => order,
                             :limit => params[:limit])
  end
end

This is a complex finder in controller, it contains fuzzy query, order and limit, it is confused and the complex finder should not be placed in controller. We should move the complex finder to model by using named_scope.

Refactor

class Post < ActiveRecord::Base
  named_scope :matching, lambda { |column , value|
    return {} if value.blank?
    { :conditions => ["#{column} like ?", "%#{value}%"] }
  }

  named_scope :order, lambda { |order|
    {
      :order => case order
      when "title" : "title desc"
      when "created_at" : "created_at desc"
      end
    }
  }

  named_scope :limit, lambda { |limit|
    { :limit => limit }
  }
end

class PostsController < ApplicationController
  def search
    @posts = Post.matching(:title, params[:title])
                 .matching(:content, params[:content])
                 .order(params[:order]).limit(params[:limit])
  end
end

The advantage to use named_scope is

• The code is much more readable, from the method call, we can know the complex finder includes fuzzy query of title and content, order and limit.
• Follow Skinny Controller Fat Model, this is the core principle of MVC.
• You can easily reuse the named_scope and handle complex finders by combining small named scopes.

updated: in rails 3 or newer versions, you should use scope instead of named_scope.