I have to write a trigger to prevent deleting of Apartment records if there are flat records related to it
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Here is my solution

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    trigger DoNotDeleteAccountHavingRelatedContact on Apartment__c(before delete)  
    {  
        for(Apartment__c acc : [SELECT Id FROM Apartment__c WHERE Id IN (SELECT Apartment__c.id FROM Flat__c) AND Id IN :Trigger.oldMap.KeySet()]){
            Trigger.oldMap.get(acc.Id).adderror('Account cannot be deleted');
        }
    }



Please help..Thank u in advance