Traverse Parent Object on Map Get Method

Question

I'm looking to retrieve the string value for the Inventory_Stage__c field in the following logic. The code is incomplete, but I'll post the relevant stuff of what I have. Any help in doing this would be greatly appreciated. Thanks in advance! I'll explain what I think is happening here - I'm a bit new to this so if my logic is wrong here, please advise.

Here I'm building a map of all the Transaction__c records with a start date from this week (Renamed to Production Orders or Production Shifts elsewhere). I am setting the key to the Name of the Inventory_Stage__c and the value to the Transaction__c object.

Map<String, Transaction__c> thisWeeksShifts = new Map<String, Transaction__c> ();
for (Transaction__c p : [SELECT Id, Name, Inventory_Stage__r.Name FROM Transaction__c WHERE Shift_Start__c = THIS_WEEK]) {
    String p_name = p.Inventory_Stage__c;
    thisWeeksShifts.put(p_name, p);
}  

And here I'm trying to loop over all the Inventory Transactions in the trigger and get the Production Shift for which the Inventory_Stage__c in the Production Shift (Transaction__c) matches that of the Inventory_Stage__c in the Inventory Transaction so I can assign it as the parent in the new record I'm creating. I'll also eventually want to assign the Inventory_Stage__c of the cloned inventory transaction to the inventory stage of the current parent Transction__c object.

for(Inventory_Transaction__c invTrans : Trigger.new) {
    if (invTrans.Type__c == 'Out') {
        SObject nextProductionOrder = (String) thisWeeksShifts.get(invTrans.getSobject('Production_Shift__r').get('Inventory_Stage__c'));
        system.debug(thisWeeksShifts.size());
        Inventory_Transaction__c invTransNew = invTrans.clone(false, true, false, false);
        invTransNew.Type__c = 'In';
        toClone.add(invTransNew);
        }
    }

Answer 1

Ok, someone will correct me if I am wrong as I do not use that method very often and I can only get it to work when including the __r.Name in the query which the trigger does not have. I do not think you can use it in this scenario.

You are going to have to get a Set for all the Production_Shift__c and the create a map to use for the values

i.e.

Set<ID> parentIDs = New Set<ID>();
for(Inventory_Transaction__c invTrans : Trigger.new) {
    if(invTrans.Production_Shift__c != null) parentIDs.add(invTrans. Production_Shift__c);
}

Map<ID,Production_Shift__c> shitMap = New Map<ID,Production_Shift__c>([Select ID, OtherFields From Production_Shift__c Where ID IN :parentIDs]);

Then cycle through the trigger records again and use the map to get your values

   for(Inventory_Transaction__c invTrans : Trigger.new) {

     If(shiftMap.containsKey(invTrans.Production_Shift__c){ //If is probably not necessary as they should be there
             //Do your stuff
     }

}