Trigger to Avoid Duplicate Combination of Parents in Junction Object

This trigger will help you to avoid duplicate entries on the Junction object with the same set of parent records.

For Ex: Let's consider we have three objects Parent1, Parent2, and Child1(Junction Object between Parent1 and Parent2).

• We have inserted a record in Junction Object (CH1) with values as a parent1 record (P1) and Parent2 Record (P2). 
• If we try to enter another Child record with again same parent combination i.e., P1 and P2 then the below trigger throw an error message saying duplicates not allowed.

Trigger:

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trigger AvoidDuplicateOnJunctionObject on Child1__c (before insert,before Update) { set<Id> Parent1Set = new set<Id>(); set<Id> Parent2Set = new set<Id>(); Set<Id> AvailableParent1Set = new set<Id>(); set<Id> AvailableParent2Set = new set<Id>(); for(Child1__c vp : trigger.new){ Parent1Set.add(vp.Parent1__c); Parent2Set.add(vp.Parent2__c); } for(Child1__c vp : [Select id, Parent1__c, Parent2__c from Child1__c WHERE Parent1__c IN: Parent1Set AND Parent2__c IN: Parent2Set]){ AvailableParent1Set.add(vp.Parent1__c); AvailableParent2Set.add(vp.Parent2__c); } for(Child1__c vp : trigger.new){ if(AvailableParent1Set.contains(vp.Parent1__c) && AvailableParent2Set.contains(vp.Parent2__c)){ vp.adderror('Duplicate Combination Values are Not Allowed'); } } }