Using count() in soql in apex Trigger.new

Question

List<CustObjChild__c> cItems = [SELECT COUNT() FROM CustObjChild__c 
             WHERE MyParentId__c IN :Trigger.new];

Getting the error: Illegal assignment from Integer to List. In Salesforce Apex Trigger.

Trigger is in the child object which has a Master-Detail relationship.

Any pointers? Thanks!

---

I had tried with Integer earlier and tried again now with Integer, but when executing the trigger, keeps getting the error:

Line: 19, Column: 1 System.DmlException: Insert failed. execution of AfterInsert caused by: System.QueryException: Invalid bind expression type of CustObjChild__c does not match domain of foreign key Class.

Answer 1

To get the children per parent, you want to group by the parent ID, and use COUNT(ID) instead of COUNT(). Also, the return type will then be an AggregateResult, not a "child object."

Map<Id, AggregateResult> counts = new Map<Id, AggregateResult>([
  SELECT MyParentId__c Id, COUNT(ID) sum
  FROM CustObjChild__c
  WHERE MyParentId__c = :Trigger.new
  GROUP BY MyParentId__c
]);

You can then get the values for each record:

for(MyParent__c record: Trigger.new) {
  Integer childRecordCount = (Integer)counts.get(record.Id)?.get('sum');
  if(childRecordCount == null) {
    childRecordCount = 0;
  }
  // ...
}

Answer 2

COUNT in SOQL return Integer.

Integer intCount = [SELECT COUNT() FROM CustObjChild__c WHERE MyParentId__c IN :Trigger.new];