Collection of List keyed by sObject field

Is it possible to have a Map (or other collection) of sObjects list, keyed by one lookup field of the same sObject.

Something like Maps of sObjects (below code doesn’t works):

Map<Id, List<Custom__c>> itemsMap = new Map<Id, List<Custom__c>>([SELECT Lookup_To_Custom__r.Id, Name FROM Custom__c]);

Note: this gives the following error Invalid initial type LIST<Custom__c> for MAP<Id,LIST<Custom__c>>

Were the Idin Map<Id, List<Custom__c>> is a reference to the Lookup_To_Custom__r.Id field.

Answers:

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Method 1

The special map constructor only handles the case of creating a map where the key is the ID and the value is the SObject.

To do what I think you want to do requires a loop:

Map<Id, List<Custom__c>> m = new  Map<Id, List<Custom__c>>();
for (Custom__c c : [
        SELECT Lookup_To_Custom__c, Id, Name
        FROM Custom__c
        WHERE Lookup_To_Custom__c != null
        ORDER BY Name
        ]) {
    List<Custom__c> l = m.get(c.Lookup_To_Custom__c);
    if (l == null) {
        l = new List<Custom__c>();
        m.put(c.Lookup_To_Custom__c, l);
    }
    l.add(c);
}

Note that using the foreign key field Lookup_To_Custom__c is a little more direct than going through the reference and taking the ID Lookup_To_Custom__r.Id.

Method 2

This is certainly possible. You just need to build the map yourself instead of trying to create it form a a single query.

As an example lets say we have a custom lookup on the contact record to another contact, say ‘Emergency Contact’. Its possible that a person is listed as the emergency contact for more 1 person, so we could create a map where the key is the contact id, and the value is actually a list of contacts in which they are the emergency contact for.

Obviously this is a bit different from your case, but the same idea and you should easily be able to adjust the code for your scenario.

Map<Id, List<Contact>> conMap = new Map<Id, List<Contact>>();
for(Contact parent : [Select Id, Name, (Select Id, Name From EmergencyContacts__r) From Contact]) {
    conMap.put(parent.Id, new list<Contact>());
    for(Contact child : parent.EmergencyContacts__r){
         conMap.get(parent.Id).add(child);
    } 
}
system.debug('My Map: ' + conMap);

You now have a map where a contact Id is the key, and a list of contacts that person is the emergency contact for is the value.

Hope that helps

EDIT

Thinking about it more, you could likely make trim this down a bit more and just do this as well.

Map<Id, List<Contact>> conMap = new Map<Id, List<Contact>>();
for(Contact parent : [Select Id, Name, (Select Id, Name From EmergencyContacts) From Contact]) {
    conMap.put(parent.Id, parent.EmergencyContacts__r);
}

system.debug('My Map: ' + conMap);

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