Saving data into custom cdr field

I created custom field “rec_name” id table “cdr”, database “asteriskcdrdb”. In this field I want to store recording name. I know i should do it by adding this line in one of .conf files, but where?
exten => s,1,set(CDR(rec_name)=${CALLFILENAME})

I can do it by sql statement, too, but i don’t know where is the file that saves details of call into database.

Help me, please, i’ll by grateful.

You can do it in /etc/asterisk/extensions.conf or in AGI snippet.