InfoPath Dev

Hi, I have an infopath form that has two external datasource I query the first one from a button to populate a dropdownlist I then want to choose an item from this dropdown list and press a button which will pass the vaule from this combobox to the second datasource. how do I set the query field for the secondary datasource on change of the dropdown (this throws a state error when I try it), or is there a better way to do it? Is there an event that fires on the refresh of a datasource?

Add a rule to your dropdown with an action that sets the query value. Add a second action that then queries the secondary data source.