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Delving Deeper into the Active Record with Ruby-on-Rails

In addition to allowing on-the-fly schema mapping, the Active Record provides a variety of services helpful to complicated table situations. Keep reading to learn more.

Author Info:
By: A.P.Rajshekhar
Rating: 5 stars5 stars5 stars5 stars5 stars / 6
November 28, 2006
  1. · Delving Deeper into the Active Record with Ruby-on-Rails
  2. · Relationship Mapping
  3. · Acts As Mapping
  4. · Mapping and Rails in the Real World

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Delving Deeper into the Active Record with Ruby-on-Rails - Mapping and Rails in the Real World
(Page 4 of 4 )

Now let's look how the mapping discussed fits into the whole Rails framework. I will be extending the existing application developed in the previous part by adding a new table and corresponding operations. I will provide a custom view part to show the use of the tree data structure. So let's get started. First comes the table. It's the CATEGORIES table, and here is the DDL for it:


create table categories (

id int not null auto_increment,

name varchar(100) not null,

/* ... */

parent_id int,

constraint fk_category foreign key (parent_id) references categories(id),

primary key (id)



Next comes the model corresponding to the table. The command to generate the model class is:


ruby script/generate model User


In the generated class I am giving the declaration acts_as_tree to tell the framework that I need the tree data structure.


    class Category < ActiveRecord::Base

acts_as_tree :order => "name"


As you can see, I am reusing the table and model discussed in the previous section. 

Next comes the controller. The command to create the controller is:


ruby scriptgenerate controller Mapping


class MappingController < ApplicationController




def list




Every other operation will be provided with a default scaffold except for listing/view. Next comes the definition of listing. Since the application is for a tree structure, the operation for creating the tree will be done in the list method.


class MappingController < ApplicationController




def list

@root = Category.create(:name => "Books")

fiction = root.children.create(:name => "Fiction")

non_fiction = root.children.create(:name => "Non Fiction")

non_fiction.children.create(:name => "Computers")

non_fiction.children.create(:name => "Science")

non_fiction.children.create(:name => "Art History")

fiction.children.create(:name => "Mystery")

fiction.children.create(:name => "Romance")

fiction.children.create(:name => "Science Fiction")




Next is the list.rhtml. Here is the code for list.rhtml.







                    <% subcat=@root. root.children.first%>


                  <!-same for all others 






That brings us to the end of this part. However, the services provided by Active Record are many. The most important are data validation, transactions and single table inheritance. These will be topics of discussion in the future. Till then...

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