Entity 'Task' is not supported for semi join inner selects

Question

I want add a default task to an opportunity when this one is close won and the opportunity hasn't any task.

I get the error compiling the below code:

Entity 'Task' is not supported for semi join inner selects

trigger ClosedOpportunityTrigger on Opportunity (after insert, after update) {
List<Task> taskList = new List<Task>();
    for (Opportunity opp : [Select Id, Name from Opportunity 
                           Where Id IN :Trigger.New AND 
                                Id NOT IN (Select WhatId from Task)]) {
    // Add a default task for this Opportunity
        If (opp.StageName == 'Closed Won') {
            taskList.add(New Task(Subject ='Follow Up Test Task',
                                  WhatId = opp.Id)); 
           }

    } //end for loop
}

Answer 1

You need to split out a separate query. As the error states, you can't use this join:

Id NOT IN (Select WhatId from Task)

One possible approach to splitting out the query:

Set<Id> opportunityIdsWithTask = new Map<Id, AggregateResult>([
    SELECT WhatId Id FROM Task
    WHERE WhatId IN :trigger.new
    GROUP BY WhatId
]).keySet();

Then just add add that as your filter instead of a join:

Id NOT IN :opportunityIdsWithTask

Answer 2

This was a solid post and ultimately helped me clean up my code.

My final code looked like this

trigger ClosedOpportunityTrigger on Opportunity(after insert, after update) {
    List<Task> taskList = new List<Task>();

    // Create a set of Opportunities with no Tasks
    Set<Id> opportunityIdsWithTask = new Map<Id, AggregateResult>([
    SELECT WhatId Id FROM Task
    WHERE WhatId IN :trigger.new
    GROUP BY WhatId
]).keySet();

    // Add an Task for each Opportunity if it doesn't already have one.
    // Iterate over Opportunities that are in this trigger but that don't have Tasks.
  for (Opportunity a : [SELECT Id,Name FROM Opportunity
                     WHERE Id IN :Trigger.New AND
                     Id NOT IN :opportunityIdsWithTask]) {

       taskList.add(new Task(Subject = 'Follow Up Test Task',
                             whatId  = a.Id));

   }


    if (taskList.size() > 0) {
        insert taskList;

   }
}

The only way I would have ever learned that tasks can't be used in inner joins is by finding this thread. I thought it was awesome because I had the same shell as OP. I added the recommended and voila!