Trigger to calculate the minimum date of Child records and populate on Parent record

Here Child -> Buyers__c

         Parent -> oneplus__c

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trigger FirstBuyer on Buyers__c (after Insert, after update, after Delete, after Undelete) { set<id>OnePlusIds = new set<Id>(); if(trigger.isInsert || trigger.isUndelete || trigger.isUpdate){ for(Buyers__c buy : Trigger.new){ OnePlusIds.add(buy.oneplus__c); } } if(trigger.isDelete || trigger.isUpdate){ for(Buyers__c buy : Trigger.old){ OnePlusIds.add(buy.oneplus__c); } } List<OnePlus__c>Parent_List = [Select Id, name, First_Buying_Date__c, (Select Id, Name, Buying_date__c From Buyers__r ORDER BY Buying_date__c ASC LIMIT 1) From oneplus__c WHERE ID IN: OnePlusIds]; List<OnePlus__c>Update_Parent_List = New List<OnePlus__c>(); for(Oneplus__c oneplus : Parent_List){ oneplus.First_Buying_Date__c = oneplus.Buyers__r[0].Buying_date__c; Update_Parent_List.add(oneplus); } Update Update_Parent_List; }