How to filter items by field with migxdb

Hello, I was wondering if it was possible to filter elements by fields. Basically, I would like to implements filters to my migxdb grid so it loads only the desired elements. Can we implement our own custom queries and if so where do I write my query.

Thanks

Mark

two examples for textfield - filters:
http://rtfm.modx.com/display/ADDON/MIGXdb.Create+a+basic+gallery-management+from+scratch+with+MIGXdb#MIGXdb.Createabasicgallery-managementfromscratchwithMIGXdb-Addingasearchfieldtoourgrid
http://rtfm.modx.com/display/ADDON/MIGXdb.Create+doodles+manager+with+help+of+MIGXdb#MIGXdb.CreatedoodlesmanagerwithhelpofMIGXdb-dbfilters