salesforce compare standard obj and custom object and assign value - triggers

Question

I have custom obj "A" and Standard obj Case. Case standard obj has lookup to custom obj "A". there is a field between the two objects called Customer_ID__c. I wrote a Trigger (before Insert, Before Update) to associated the case record to the correct existing custom obj "A" record if "Case.Custom_Id__c" match the one in the Custom obj "A". Unfortunate it is not happening and I'm not sure where to look.
trigger IAACaseRelateASAP on Case (before insert, before update) {
Id recordtypes = [Select Id, name
From RecordType
Where SobjectType = 'Case'
AND Name = 'I Buy'
LIMIT 1].Id;
Set<String> casId = new Set<String>();
for(Case cs : Trigger.new)
{
if(cs.RecordtypeId == recordtypes && cs.Type == 'Contact Me')
{
if(cs.custm_Obj_A_Name__lookupfield__c == null && (cs.Customer_ID__c != null || cs.Customer_ID__c !='0'))
{
casId.add(cs.Customer_ID__c);
}
}
}
system.debug('Case Set Ids' + casId);
List<A__c> aList = [Select Customer_ID__c, Id
From A__c
Where Customer_ID__c IN: casId
AND
A__c != 'Provider'];
System.Debug('equals' + aList);
Map<String, A__c> aMapId = new Map<String, A__c>();
for(A__c aAcct : aList)
{
aMapId.put(aAcct.Customer_ID__c, aAcct);
}
for(Case cas : Trigger.new)
{
if(cas.RecordtypeId == recordtypes && cas.Type == 'Contact Me')
{
if(cas.custm_Obj_A_Name__lookupfield__c == null && (cas.Customer_ID__c != null || cas.Customer_ID__c !='0'))
{
if(aMapId.containsKey(cas.Customer_ID__c))
{
A__c aAcct = aMapId.get(cas.Customer_ID__c);
System.Debug('Case IAA ASAP Account value: ' + asapAcct);
}
}
}
}
}

Answer 1

It might be best when looping through your cases to build your set of Customer_ID__c ids to also build a List of cases with customer ids so that you don't have to loop through the entire new list a second time. There are a couple other issues with the trigger in general but I'll ignore those and just focus on what you asked. Think your issue is that you don't actually set the case field in this area:
if(aMapId.containsKey(cas.Customer_ID__c))
{
A__c aAcct = aMapId.get(cas.Customer_ID__c);
System.Debug('Case IAA ASAP Account value: ' + asapAcct);
}
It should be :
if(aMapId.containsKey(cas.Customer_ID__c))
{
cas.custm_Obj_A_Name__lookupfield__c = aMapId.get(cas.Customer_ID__c).Id;
}