How To Group Records By Value In A Specific Field?

Question

I have a List of records of type abc__c. abc__c is a custom object. The record has a field called code__c. What I am trying to accompolish is to group the records in the list based on the code__c.

For example the list might have :

Code__c     abc__c record
30       (Record 1)
40       (Record 2)
40       (Record 3)
50       (Record 4)
30       (Record 5)

I want to group these records based on Code__c in a Map. There could be any number of code__c which is populated dynamically. It is not static. how can I loop through the list and store the records as a list for a particular code__c(Id) in Map?

Answer 1

You can use a List as a Value for a Map.

Map<Integer, ABC__c[]> codeToABC = new Map<Integer, ABC__c[]>();
for(ABC__c record: abcRecords) {
    if(codeToABC.containsKey(record.Code__c)) {
        // Code already in map
        codeToABC.get(record.Code__c).add(record);
    } else {
        // Code is not yet in Map
        codeToABC.put(record.Code__c, new List<ABC__c> { record });
    }
}

You can swap out Integer for another data type (e.g. if Code is actually a String).

Answer 2

This is a pretty common pattern. So common it is worthwhile (and pretty straightforward) to write a utility to do so. Here is an example signature:

public class GroupBy
{
    public static Map<String, List<SObject>> strings(List<SObject> records, SObjectField field)
    {
        Map<String, List<SObject>> grouped = new Map<String, List<SObject>>();
        for (SObject record : records)
        {
            String value = (String)record.get(field);
            if (!grouped.containsKey(value))
                grouped.put(value, new List<SObject>());
            grouped.get(value).add(record);
        }
        return grouped;
    }
}

You might want to write signatures for other types, such as Integer, Id, Date, etc. Then any time you need to perform functionality like this, you can just call:

Map<String, List<Abc__c>> codeToAbcs = GroupBy.strings(abcRecords, Abc__c.Code__c);

Answer 3

You can put lists in maps, then list in your map by the group. For example, I put together a quick grouping of accounts by a field here. Just replace the objects and fields to loop through

    List<account> acctList = [Select Name, Client_Executive__c FROM Account LIMIT 100];
set<String> repList = new set<String>();
for (Account acct: acctList) {
    repList.add(acct.Client_Executive__c );
}

Map<string, List<account>> acctMap = new map<string, List<account>>();
for (string rep:repList ) {
    List<Account> repAcctList = new List<Account>();
    for (Account acct: acctList) {
        if (rep == acct.Client_Executive__c) {
            repAcctList.add(acct);
        }
    } 
    acctMap.put(rep, repAcctList);
}
system.debug(acctMap);