Extract Child Records in SOQL Query into Apex Map?

Question

I have a SOQL query that pulls a list of accounts and associated contact records.

Is there a way to get the child contact records into a map without looping over the accounts and then looping over the contacts?

List<Account> accountsWithContacts = [SELECT ID, Name, (SELECT Id, Name FROM Contacts__r)
FROM Account];

Map<Id, Contact> contactMap = new Map<Id,Contact>();

If there are 7 contacts returned for a given Account I am trying to figure out the map syntax to extract just the child contacts into a map of of their own.

Answer 1

You can not get the map of contactId to Contact in an inner Query directly.

However, if you want to avoid looping over Account you can use a second query to get contacts by AccountId and store it in Map.

Example:

List<Account> accounts = [Select Id, Name From Account];
Map<Id, Contact> contacts = new Map<Id, Contact>([Select Id, Name From Contact Where AccountId in :accounts]);

Answer 2

As this query is Parent-child you cannot store directly from inner query as the correct map will be

Map<Id,Account> conMap = new Map<Id,Account>(accountsWithContacts);

so there is no way to fill your map directly from innerquery

instead you can make a map with accountId as a key and List as values as One account can have many contacts so the best way is to make a map like

Map<Id,List<Contact>> contactMap = new Map<Id,List<Contact>>();

and to fill this

List<Account> acList = new List<Account>([SELECT Id From Account Limit 100]);
List<Contact> conList = new List<Contact>([SELECT Id,AccountId From Contact WHERE AccountId in: acList]);

Map<Id,List<Contact>> contactMap = new Map<Id,List<Contact>>();

for(Contact con: conList){
    if(!contactMap.ContainsKey(con.AccountId)){
        contactMap.put(con.AccountId,new List<Contact>());
    }
    contactMap.get(con.AccountId).add(con);
}
System.debug('ContactMap :'+contactMap);