How to Add Custom Field in the Shipping Address of Onepage Checkout in Magento

Sometimes we need to add the custom fields in to the Onepage checkout process, here I am adding a custom field "comments" into the "Shipping Information" tab of Onepage checkout process and storing the values into the tables
"sales_flat_quote_address" & "sales_flat_order_address".

Please see the example below for better understanding:

Step1: Add Custom Field "Comments" into Shipping Address tab of the Onepage Checkout in Frontend.
app\design\frontend\[Package_name]\[Theme_name]\template\checkout\onepage\shipping.phtml

<li class="control">
  <div>
     <label for="shipping:comments"><?php echo $this->__('Comments') ?  ></label><br />
    <input type="text"  name="shipping[comments]" id="comments" title="Comments" value="" class="input-  text" maxlength="20" />
 </div>
</li>

Creating custom module name as "Customization"
Step2: Create app\etc\modules\Namespace_ Customization.xml
<Namespace_Customization>
            <active>true</active>
            <codePool>local</codePool>
</Namespace_Customization>

Step3: create a xml file
app\code\local\Namespace\Customization\etc\ config.xml
<config>
    <modules>
        <Namespace_Customization>
            <version>0.1.0</version>
        </Namespace_Customization>
    </modules>
    <global>
  <models>
            <namespace_customization>
                <class>Namespace_Customization_Model</class>
                <resourceModel>namespace_customization_mysql4</resourceModel>
            </namespace_customization>
   <namespace_customization_mysql4>
    <class>Namespace_Customization_Model_Mysql4</class>
   </namespace_customization_mysql4> 
        </models>
  <resources>
            <namespace_customization_setup>
                <setup>
                    <module>Namespace_Customization</module>
     <class>Namespace_Customization_Model_Mysql4_Setup</class>
                </setup>
                <connection>
                    <use>core_setup</use>
                </connection>
            </namespace_customization_setup>
            <namespace_customization_write>
                <connection>
                    <use>core_write</use>
                </connection>
            </namespace_customization_write>
            <namespace_customization_read>
                <connection>
                    <use>core_read</use>
                </connection>
            </namespace_customization_read>
        </resources>
  <blocks>
            <namespace_customization><class>Namespace_Customization_Block</class></namespace_customization>
        </blocks>
        <helpers>
            <namespace_customization><class>Namespace_Customization_Helper</class></namespace_customization>
        </helpers>      
 <fieldsets>
      
<sales_convert_quote_address>
   <comments><to_order_address>*</to_order_address></comments >
  </sales_convert_quote_address>
  <sales_convert_order_address>
  < comments ><to_quote_address>*</to_quote_address></comments >
              </sales_convert_order_address>
 </fieldsets>
    </global>
    <default>
        <cart>
            <email>
                <email_identity>general</email_identity>
                <email_template>cart_email_email_template</email_template>
            </email>
        </cart>
    </default>
    <frontend>
        <routers>
            <checkout>
                <args>
                    <modules>
                        <namespace_customization after="Mage_Checkout">Namespace_Customization</namespace_customization>
                    </modules>
                </args>
            </checkout>
        </routers>
        <routers>
            <customization>
                <use>standard</use>
                <args>
                    <module>Namespace_Customization</module>
                    <frontName>customization</frontName>
                </args>
            </customization>
        </routers>
 
    </frontend> 
</config>

Step4: create a sql setup file app\code\local\Namespace\Customization\sql\namespace_customization_setup\mysql4-install-0.1.0.php
Add the content below in file.

<?php
$installer = $this;
/* @var $installer Mage_Sales_Model_Mysql4_Setup */

$entityAttributesCodes = array(
    'comments' => 'varchar'  
);
foreach ($entityAttributesCodes as $code => $type) {
    $installer->addAttribute('quote_address', $code, array('type' => $type, 'default' => '0', 'user_defined' => false));
    $installer->addAttribute('order_address', $code, array('type' => $type, 'default' => '0', 'user_defined' => false));
    }

Step 5: Override the model file into your local app\code\local\Namespace\Checkout\Model\Type\Onepage.php
and add the below content.

class Namespace_Checkout_Model_Type_Onepage extends Mage_Checkout_Model_Type_Onepage
{
    /**
     * Save checkout shipping address
     *
     * @param   array $data
     * @param   int $customerAddressId
     * @return  Mage_Checkout_Model_Type_Onepage
     */
    public function saveShipping($data, $customerAddressId)
    {
        if (empty($data)) {
            return array('error' => -1, 'message' => $this->_helper->__('Invalid data.'));
        }
        $address = $this->getQuote()->getShippingAddress();
        ...................
            .....................Intermediate code ........
                ........................
             
        if (!empty($customerAddressId)) {
            $customerAddress = Mage::getModel('customer/address')->load($customerAddressId);
            if ($customerAddress->getId()) {
              ...................
                ..................... Intermediate code ........
                ........................
                if ($addressErrors !== true) {
                    return array('error' => 1, 'message' => $addressErrors);
                }
                // comments: set comments field value              
                $address->setSpecDelComment2(empty($data['comments']) ? '' : $data['comments']);          
            }
        } else {
            $addressForm->setEntity($address);
            ...................
                ..................... Intermediate code ........
                ........................
            $addressForm->compactData($addressData);          
            // comments: set comments filed value
            $address->setSpecDelComment2(empty($data['comments']) ? '' : $data['comments']);
        }

        ...................
                ..................... Intermediate code ........
                ........................
        $this->getQuote()->collectTotals()->save();
        ...................
                ..................... Intermediate code ........
                ........................

        return array();
    }

Step 6: Refresh the Cache & ur done.