Delving Deeper into the Active Record with Ruby-on-Rails - Mapping and Rails in the Real World
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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"
end
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
scaffold:category
def list
end
end
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
scaffold:category
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")
end
end
Next is the list.rhtml. Here is the code for list.rhtml.
<ul>
<li>
<%=@root.name%>
<ul>
<li>
<% subcat=@root. root.children.first%>
<%=subcat.name%>
<!-same for all others
</li>
</ul>
</li>
</ul>
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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