Trigger to update child object values on parent object field (concatenated with unique)

Question

I need help to get the unique names from the child object records and concatenate them on the text field on the parent object. I was able to concatenate but unable to unique the values. As an example, if there are 4 child records (apple, mango, mango, apple) to the parent record. I want to update as apple; mango on the parent object field.

list<id> parentIDstoUpdate = new list<id>();
    if(Trigger.isAfter){
        if(Trigger.isInsert || Trigger.isUpdate ){
            for (ChildObject__c co : Trigger.new) {
                parentIDstoUpdate.add(co.ParentObject__c);       
            }
        }  
    }

    map<id,string>  mapParentIdtoString = new Map<ID,String>();
    string str;

    for (ChildObject__c co: [SELECT Fruit__c, ParentId FROM ChildObject__c WHERE ParentId =:parentIDstoUpdate ]){
        if(mapParentIdtoString.containsKey(co.ParentId)){
            str = mapParentIdtoString.get(co.ParentId);
            str = str + '; ' + co.Fruit__c;  
            mapParentIdtoString.put(co.ParentId,str);
        }else{
            mapParentIdtoString.put(co.ParentId,co.Fruit__c);
        }
    }

    list<ParentObject__c> recordstoUpdate = new list<ParentObject__c>();
    for (ParentObject__c p: [SELECT ID, Parent_Field__c FROM ParentObject__c WHERE id =:recordstoUpdate ]){ 
        p.Parent_Field__c = mapParentIdtoString.get(p.id);
        recordstoUpdate.add(p);
    }

    if (recordstoUpdate != null && recordstoUpdate.size() >0){
        update recordstoUpdate;
    }
}

Answer 1

You should be able to use a set for something like this.

When you're looping through your child objects:

Set<String> uniqueFruits = new Set<String>();

for (SObject record : recordList) {
    uniqueFruits.add(record.field__c);
}

The set will automatically de-dup the strings. Then add them to the parent object:

String textField = '';

for (String s : uniqueFruits) {
    textField = textField + s + ',';
}

textField = textField.left(textField.length() -1); // Remove the last comma