Trying to update Values in Standard Object from metaData - triggers

Question

I am using Trigger isBefore
In System.debug(opp.get(metaData.get(0).Opportunity_Field_Name__c), it is showing correct Values but not Updating in Opportunity Object
Below is Trigger and its Apex Class Trigger
Trigger
trigger MetadataObjectFieldMapping on Opportunity (before insert, before update)
{
if(Trigger.isInsert || Trigger.isUpdate )
{
MetadataObjectFieldMappingHandler oppHandler = new MetadataObjectFieldMappingHandler();
oppHandler.Show(Trigger.new);
}
}
And Apex Class
public class MetadataObjectFieldMappingHandler {
List<String> strAccField = new List<String>();
//Getting List of MetaData Values
List<Object_Field_Mapping__mdt> metaData = new List<Object_Field_Mapping__mdt>
([SELECT Account_Field_Name__c,
Opportunity_Field_Name__c
FROM Object_Field_Mapping__mdt]);
//Function to check if Field Name Exists in Object or not
public Boolean hello(String objName, String fieldName)
{
Boolean temp = False;
//Creating Schema to get all fields from Account and Opportunity Object
Map<String, Schema.SObjectField> accFields = Schema.getGlobalDescribe().get(objName).getDescribe().fields.getMap();
for(Schema.SObjectField field : accFields.values())
{
strAccField.add(field+'');
}
//Calling Account and Opportunity Object in fieldName
if(strAccField.contains(fieldName)){
System.debug('PASS '+fieldName);
temp = true;
}
return temp;
}
public void Show(List<opportunity> newOppList)
{
Boolean test1 = hello('Account',metaData.get(0).Account_Field_Name__c);
Boolean test2 = hello('Opportunity',metaData.get(0).Opportunity_Field_Name__c);
//If both Field Value exists
if(test1 && test2){
//Getting value from Opp using dynamic Query
String query = 'Select Account.'+metaData.get(0).Account_Field_Name__c+', '+metaData.get(0).Opportunity_Field_Name__c+' from Opportunity where Id IN : newOppList ';
List<Opportunity> oppList =database.query(query);
for(Opportunity opp : oppList){
opp.put(
metaData.get(0).Opportunity_Field_Name__c,
opp.Account.get(metaData.get(0).Account_Field_Name__c)
);
System.debug(opp.get(metaData.get(0).Opportunity_Field_Name__c));
}
}
}
Can you please tell me, why Value is not Updating in Opportunity Object while it showing in Debug Logs..?

Answer 1

Multiple fails here I think.
Apex is case-insensitive when you do if('a' == 'A'). But when comparing Strings in collections (Lists, Sets, Map keys) it suddenly becomes case-sensitive.
List<String> fields = new List<String>{'Id', 'Name'};
System.debug(fields.contains('name')); // false
(this should have been a Set<String> by the way, for performance and logical readability). So I suspect something's fishy there, in case. You didn't show your metadata but check this one (you have only 1 row now, right? If you have more than one - we'll your metaData.get(0) essentially returns a random row).
I don't like the cast from Schema.SObjectField to String either.
Next: String query = 'Select Account.'+metaData.get(0).Account_Field_Name__c+', '+metaData.get(0).Opportunity_Field_Name__c+' from Opportunity where Id IN : newOppList ';
This has a chance of working in before update. But for sure not in before insert. Nothing's in database yet, your query will return zero results. You have to loop through trigger.new, collec AccountIds, query Accounts (directly, not via Opportunity table) and then make final loop that writes data.
You passed newOppList to Show(). If you want to get the save to database for free - you should modify values on the original, on newOppList. Instead you modify the in-memory results of query (oppList). Nothing will happen to them, they'll be discarded. If you want to save them, you'd have to do it manually (but then you risk entering a loop of update triggers and SF will stop you).
You sure this has to be code? Sounds like a job for workflow or process builder. Or make them formula fields so you always display fresh value instead of such copying... When something changes on Account it won't automatically cascade down to all opps unless you make next trigger/process builder...