I'm working on being able to translate a part number (stored as a string) from one extenal system to another.  I have a custom table setup each record will have both part numbers and would like the user to be able to enter either number and have SF provide the other.  If a number is not found in the table, query an external system (using HTTP callout) to retrieve it then save it in the table so future requests for the same number don't have to callout to the external system (essentially add records on the fly).

Here's the flow:  user enters a part number from system A.  This field is a string.  Apex queries the custom table to see if it is saved.
 

List<Part__c> p = new List<Part__c>();
p = [Select Return__c, Regional_Scrap__c, Return_Factory__c, Factory_Approval__c, Name, Description__c, SAP_Number__c From Part__c Where Name = :li.Part_Number__c];

First time part number is used, the query will return 0 records [p.size()=0].  Apex will then use the HTTP callout to query the external system to get the other part number.  Callout is working fine.  Response gets parsed into a List<String> and the field value set.

HTTPResponse res = http.send(req);
bodyval = res.getBody();
list<string> dfresults = bodyval.split('\t', 10);
li.SAP_Number__c = dfresults[4];

Part__c Prt = new Part__c();
Prt.Name = li.Part_Number__c;
Prt.SAP_Number__c = li.SAP_Number__c;
insert Prt;

p = [Select Return__c, Factory_Approval__c, Regional_Scrap__c, Return_Factory__c, Description__c, Name, SAP_Number__c From Part__c Where SAP_Number__c = :li.SAP_Number__c];