access individual field values from an object element in a list

Question

What I am trying to do is to update a child object field (downloads__c) in each of the child records in the child object (Data__c) record collection, with the value of a field (downloads__c) from its parent object (Parent__c). The parent object also has the field count__c.

Once all the records are updated in the child collection, do a dml insert after the loop. Can't seem to get this to work. Hope I have explained correctly.

jsonBody = '[{"count__c":"45","downloads__c":"30"},{"count__c":"40","downloads__c":"20"}]'; // child records to be updated

// Querry parent 
List<Parent__c> parentList = [SELECT Id, Name FROM Parent__c 
                WHERE count__c IN :countList]; // countList will have unique values of count__c from the child json records

// child object Data__c records coming from json
List<Data__c> dList = (List<Data__c>) System.JSON.deserialize(jsonBody, List<Data__c>.class);

Map<String, List<Data__c>> mapCountByData = new Map<String, List<Data__c>>();
for(Data__c d : dList) {
    if(mapCountByData.containsKey(d.count__c)) {
       List<Data__c> lstData = mapCountByData.get(d.count__c);
       lstData.add(d);
       mapCountByData.put(d.count__c, lstData);
    } else {
        mapCountByData.put(d.count__c, new List<Data__c> { d });
    }
}

Answer 1

Maps are the way to go here,

List<Data__c> dList = (List<Data__c>) System.JSON.deserialize(jsonBody, List<Data__c>.class);
Map<String, List<Data__c>> mapCountByData = new Map<String, List<Data__c>>();
for(Data__c d : dList) {
    if(mapCountByData.containsKey(d.count__c)) {
       List<Data__c> lstData = mapCountByData.get(d.count__c);
       lstData.add(d);
       mapCountByData.put(d.count__c, lstData);
    } else {
        mapCountByData.put(d.count__c, new List<Data__c> { d });
    }
}

Then you can get all the counts using,

set<integer> setCounts = mapCountByData.keyset();

this will also not have duplicates and you can use the set in the IN clause in SOQL.

Later you can also make use of the mapCountByData to get the list of Data__c records.

Answer 2

You can use a Set or a List. Sets are preferred.

Set<Integer> values = new Set<Integer>();

for(Data__c d : dList){
  values.add(d.count__c);
}

Note that Count__c appears to be a number, so we use a number data type. If the actual field is a String, conversion is necessary:

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

for(Data__c d : dList){
  values.add(String.valueOf(d.count__c));
}