How to Create Select field in Magento

I want to create a select field in magento system.xml, and calling a model from that but getting error. My code开发者_运维百科 is...

<banner_type translate="label">
                            <label>Banner Type</label>
                            <frontend_type>select</frontend_type>
                     <source_model>banners/bannertype</source_model>
                            <sort_order>3</sort_order>
                            <show_in_default>1</show_in_default>
                            <show_in_website>1</show_in_website>
                            <show_in_store>1</show_in_store>
                    <comment>(If you select banner type, then display with that type!)</comment>
 </banner_type>

Error:

Fatal error: Call to a member function toOptionArray() on a non-object in /var/www/html/imran/magento/app/code/core/Mage/Adminhtml/Block/System/Config/Form.php on line 399

Magento attempting to instantiate a model class

Mage::getModel('banners/bannertype')

so it can it's toOptionArray method. However, in your system banners/bannertype doesn't resolve to a valid model class, so PHP complains.