I am working in Magento 1.9 , I have made a multiselect field in system.xml . When i select some fields in multiselect and click on save config it works fine , but when i did not select any field it should save empty string in DB but it stores the previous values . see my code in system.xml.
<onerateserviceOptions>
<label>One Rate </label>
<frontend_type>multiselect</frontend_type>
<source_model>FedExSmall/Source_FedExSmallIoneRateServices</source_model>
<sort_order>12</sort_order>
<show_in_default>1</show_in_default>
<show_in_website>1</show_in_website>
<show_in_store>1</show_in_store>
</onerateserviceOptions>
Note : That issue only occur in case when i did not select any field . Looking for kind help . Thank You in advance.