trigger

gistfile1.txt

trigger PullOrderSiteAccount on Orders__c (before insert) {

// Collect site codes for duplicate checking
Set<String> codes = new Set<String>();
  for (Orders__c order : trigger.new) codes.add(order.Site_Code__c);

// Find accounts with matching site codes    
Map<String, List<Account>> siteAccounts = new Map<String, List<Account>>();
  for (Account record : [SELECT Site_Code_ID__c FROM Account WHERE Site_Code_ID__c IN :codes]
    )   
// Match the two in a map
{
    if (!siteAccounts.containsKey(record.Site_Code_ID__c))
        siteAccounts.put(record.Site_Code_ID__c, new List<Account>());
    siteAccounts.get(record.Site_Code_ID__c).add(record);
}

// Only update the lookup if one account matches, else do not update
for (Orders__c order : trigger.new)
{
    List<Account> matchingAccounts = siteAccounts.get(order.Site_Code__c);
    Id parentId = (matchingAccounts != null && matchingAccounts.size() == 1) ?
        matchingAccounts[0].Id : null;

        order.Site_Account__c = parentId;
}
}