How to get related list from Trigger.new variable?

trigger TestTrigger on Opportunity (before update){
   for(Opportunity o: Trigger.new){
      List<Child_Object__c> childObjectList = o.Child_Object__r;
      System.debug('childObjectList size is '+ childObjectList.size());
   }
}

In the above code the childObjectList size is always printing as zero even though the opportunity object has child objects.

Does it mean that I need to fire SOQL query to get the related list? I thought trigger.new variable has related list? Is my understanding wrong?

Answers:

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Method 1

Trigger.new only contains a list of the sObject records that the trigger is running on, in this case Opportunities. If you want to get the related records from a different sObject then you’ll need to perform a SOQL query:

for (Opportunity o : [SELECT Id, (SELECT Id FROM Child_Objects__r) FROM Opportunity WHERE Id IN :trigger.newMap.keySet()])

Or query the related records directly:

for (Child_Object__c childRecord : [SELECT Id FROM Child_Object__c WHERE Opportunity__c IN :trigger.newMap.keySet()])

Method 2

Yes you need to fire SOQL query.”Trigger.new” only returns a list of the new versions of the sObject records under context but not the related list records.

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