Hello.  I am fairly new to Apex programming and have been trying to cobble together a solution via an earlier post, in addition to other ideas seen on this board.  That said, I am still having issues.  I would be very appreciative for any guidance.

Specifically, I have two custom objects - one each for customers (Customer__c) and deliveries (Deliveries__c).  I am trying to update a field (NumberOfDeliv__c) in the customers object to reflect the current number of deliveries that have a status of “pending” for each customer.  This value is derived by totaling the number of occurrences of “Deliveries__c” records for each customer that have a “DelivStatus__c” value of “pending”.  Due to a separate reason, I must do this update via Apex.

I am doing this in two phases. 1) Doing an AggregateResult function to total the number of pending deliveries for each customer…and then 2) Using a map to update the respective value for each customer. 

My initial problem is that I am getting an error message "Initial term of field expression must be a concrete SObject: MAP<Id,Customer__c>".

Any suggestions on why I am getting this error - or also alternate code ideas to arrive at my overall goal - would be very appreciated.  Thanks.

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trigger CountCustomers on Deliveries__c (before insert) {

       
    Map <String, Integer> DeliveryCount = new Map <String, Integer> ();
   
    String DelivID = null;
    Integer nbr = null;
   

list<string> DelivList = new list<string>();
for (Deliveries__c dlv : Trigger.new){
if (dlv.DelivStatus__c != null)
DelivList.add(dlv.DelivStatus__c);
}

//Counting the occurrences of pending deliveries for each cusotmer (using CustomerID__c)

//The CustomerID__c field is simply a string representation in the Deliveries__c object of the customer record ID

    for (AggregateResult agr: [SELECT  CustomerID__c, count(Id) NumOfCust FROM Deliveries__c
        WHERE CustomerID__c in :DelivList
         AND (DelivStatus__c = 'pending')
           GROUP BY CustomerID__c]){
           
        DelivID = string.valueOf(agr.get('CustomerID__c'));
        nbr = integer.valueOf(agr.get('NumOfCust'));
        DeliveryCount.put (DelivID, nbr);   
        }

List<Id> Cstmrs = new List<Id>{};

//Creating Map of CustomerObject
Map <ID, Customer__c> CustFields = new Map <ID, Customer__c>([select id, NumberOfDeliv__c from Customer__c where id in:Cstmrs]);

//Looping through the DeliveryObject 
    for(Deliveries__c Deliv : Trigger.new)
    {
    if (Deliv.CustomerID__c != null){
   
//Holding the field updates for each MenuTemsObject
    Customer__c cust = CustFields.get(Deliv.CustomerID__c);
     
              IF(cust.NumberOfDeliv__c == null || (Trigger.isupdate && cust.NumberOfDeliv__c == Trigger.oldMap.get(Deliv.id).Integer.valueOf(DeliveryCount.get(CustFields.Id))))

              cust.NumberOfDeliv__c =  Integer.valueOf(DeliveryCount.get(CustFields.Id));
             
              CustToUpdate.add(cust);
              }
    }
     
              }