Making XML in a Rails App
by Deepak Vohra | Pages: 1, 2

The default migration script consists of methods self.up and self.down. The self.up method is invoked to run a migration and create a database table. The create_table transformation of class ActiveRecord::Migration is used to create the database table catalogs. ActiveRecord uses pluralization to map a model class to a database table. The model class is singular and uppercase and the database table is plural and lowercase. In the example Ruby on Rails application, the model class is Catalog and the database table is catalogs.

Modify the migration script 001_create_catalogs.rb to create a database table and add data to the table. In the create_table transformation, create a table catalogs with columns journal, publisher, edition, title, and author, as shown in following listing:

class CreateCatalogs < ActiveRecord::Migration
  def self.up
    create_table :catalogs do |t|
  t.column :journal, :string, :limit => 255
  t.column :publisher, :string, :limit => 255
  t.column :edition, :string, :limit => 255
  t.column :title, :string, :limit => 255
  t.column :author, :string, :limit => 255

end

Catalog.create :journal => "IBM developerWorks", 
:publisher => "IBM", :edition => 
" October 2005", 
:title=> "JAXP validation",
:author=>"Brett McLaughlin"
Catalog.create :journal => "IBM developerWorks", 
:publisher => "IBM", :edition => "July 2006", 
:title=> "The Java XPath API",
:author=>"Elliotte Rusty Harold"
  
  end

  def self.down
    drop_table :catalogs
  end
end

Run the migration script with rake. rake is similar to J2EE's Ant. Rails provides a target called migrate to run migrations.

C:/ruby/databasexml>rake migrate

The database table catalogs is created in the MySQL database. Next, create a controller script, including a controller action gen_xml, to generate an XML document from the database table.

C:/ruby/databasexml>ruby script/generate
controller catalog gen_xml

The controller script app/controllers/catalog_controller.rb is created. Modify the model script to set the database table and establish a connection with the MySQL database. The database table is set with the set_table_name method of the ActiveRecord::Base class. The database connection is established with the establish_connection method of the ActiveRecord::Base class. The model script catalog.rb is shown below:

require 'active_record'

class Catalog < ActiveRecord::Base
set_table_name "catalogs" 
end

ActiveRecord::Base.establish_connection(
  :adapter  => "mysql",
  :host     => "localhost",
  :username => "root",
  :password =>    nil,      
  :database => "test"
)

In the controller script, modify the controller action gen_xml to retrieve data from the MySQL database. In the gen_xml method, create a Builder::XmlMarkup object.

@xml = Builder::XmlMarkup.new

Retrieve data from the MySQL database table catalogs, which was set in the model script, using the find(:all) method. The find(:all) method retrieves all the columns from the database table. To run a specific SQL statement to retrieve data, use the find_by_sql(sql)method.

@catalogs=Catalog.find(:all)

The complete controller script is shown below:

class CatalogController < ApplicationController
  def gen_xml
    @xml = Builder::XmlMarkup.new
    @catalogs=Catalog.find(:all)
    end
end

Modify the gen_xml.rhtml view template in the views/catalog directory to gen_xml.rxml. Delete the gen_xml.rhtml view template. In the RXML template, create an XML document from data retrieved from the database. Add an XML declaration.

@xml.instruct! :xml, :version=>"1.0"

Add root element catalogs.

@xml.catalogs{}

In the root element, iterate over the data retrieved from the database and create a catalog element for each row of data in the result set. The @catalogs variable was set in the controller script and represents the result set of data retrieved from the database. For example, retrieve the value of the journal column as follows:

for catalog in @catalogs
    @xml.catalog do
      @xml.journal(catalog.journal)
      …
      …
    end

The complete gen_xml.rxml file is shown below:

@xml.instruct! :xml, :version=>"1.0" 
 
@xml.catalogs{
  for catalog in @catalogs
    @xml.catalog do
      @xml.journal(catalog.journal)
      @xml.publisher(catalog.publisher)
      @xml.edition(catalog.edition)     
      @xml.title(catalog.title)
      @xml.author(catalog.author)
    end
  end
}

Next, run the Rails application to generate an XML document. Start the WEBrick web server if it is not already started.

C:/ruby/databasexml>ruby script/server

Invoke the gen_xml controller action with the URL http://localhost:3000/catalog/gen_xml. In the gen_xml controller action, data is retrieved from the MySQL database and the view template gen_xml.rxml is rendered. In the gen_xml.rxml template, an XML document is generated as shown in Figure 1.

Figure 1. Generating XML Document

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2. MySQL Connection
   2007-01-22 16:15:16 dvohra09
4. XmlMarkup object @xml
   2007-01-18 10:52:06 dvohra09