DBEdit refresh

I ! This is my first time here and my english isn't probably the best but i'll give it a try!
I just started with c++ and OO (before i was programing in C, Pascal and Fortran). I've read a few books already that gaved (?) me some tips but ... not enough!
I have created a simple database with 1 table and 2 fields (UserCode [is the primary key] and UserName).
I have a DBEdit for UserCode and another for UserName and i whant that when input the UserCode in DBEdit1, the UserName in BDEdit2 to be automaticaly refreshed! How can i do that?
Every time i try some thing i get a primary key violation - it's trying to save the chages in the datadase ....
Please help me! Oh .... i'm using Borland C++!
Thanks!