How to add a foreign key to a table programmatically?

Question

I am trying to add a set of new tables and I'm trying to add a foreign key to link these tables. I have looked online to try and find some information about addForeignKey() but cannot find anything.

Looking at Varien_Db_Adapter_Interface::addForeignKey() and Mage_Core_Model_Resource_Setup::getFkName() I thought I had got below setup correctly

->addForeignKey(
     $installer->getFkName('namespace_module/shop', 'area_id', 'namespace_module/area','area_id'),
     $installer->getTable('namespace_module/shop'), 
     'area_id',
     $installer->getTable('namespace_module/area'), 
     'area_id',
     Varien_Db_Ddl_Table::ACTION_CASCADE, 
     Varien_Db_Ddl_Table::ACTION_CASCADE
)  

But the exception says different

Zend_Db_Exception Object
(
[_previous:Zend_Exception:private] => 
[message:protected] => Undefined column "shop_locator_shop"
[string:Exception:private] => 
[code:protected] => 0
[file:protected] => /Users/myname/Sites/site/www/lib/Varien/Db/Ddl/Table.php
[line:protected] => 450
[trace:Exception:private] => Array
    (
        [0] => Array
            (
                [file] => /Users/myname/Sites/site/www/app/code/local/Namespace/Module/sql/namespace_module_setup/install-0.1.0.php
                [line] => 110
                [function] => addForeignKey
                [class] => Varien_Db_Ddl_Table
                [type] => ->
                [args] => Array
                    (
                        [0] => FK_SHOP_LOCATOR_SHOP_AREA_ID_SHOP_LOCATOR_AREA_AREA_ID
                        [1] => shop_locator_shop
                        [2] => area_id
                        [3] => shop_locator_area
                        [4] => area_id
                        [5] => CASCADE
                        [6] => CASCADE
                    )

            )

Basically how do I go about adding a foreign key to my table (I have added a screen shot of what I am trying to achieve)