trigger PullProjectNoOpportunity on SAP_Sales_Order__c (before update) {

Set<String> codes = new Set<String>();
for (SAP_Sales_Order__c o : trigger.new) codes.add(o.SAP_Project_NO__c);

Map<String, Opportunity> pronumOpportunities = new Map<String, Opportunity>();
for (Opportunity a : [
    SELECT Cilioproject__c FROM Opportunity
    WHERE Cilioproject__c IN :codes
]) pronumOpportunities.put(a.Cilioproject__c, a);

       for (SAP_Sales_Order__c o : Trigger.new)
    {
        Opportunity pronumOpportunity = pronumOpportunities.get(o.SAP_Project_NO__c);
       Id parentId = (pronumOpportunity == null) ? null : pronumOpportunity.Id;
        o.project__c = parentId;
    }
}

@isTest
private class testPullProjectNoOpportunity {
    static testMethod void validateLookupPopulation() {
        SAP_Sales_Order__c SO = new SAP_Sales_Order__c(name='9999999');
                SO.SAP_Project_No__c = '30157';
        insert SO;
    }
}