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Your entity query works fine (without aggregate). dpm() doesn't work in D8 for content entities, try kint() instead, but that will also be misleading.

Your last attempt was almost correct: -&gt;value is a property (a magic property, actually), not a method. So this should work fine:

$holidayType-&gt;field_activityid-&gt;value


As referenced already in a few questions, have a look at http://wizzlern.nl/drupal/drupal-8-entity-cheat-sheet; it's a bit old but it's still a great overview of the Content Entity (Field) API.

If you do want to use dpm()/kint(), then always run it on $entity-&gt;toArray(). That will give you an array structure of the fields, you can translate that 1:1 to object properties and it will work fine. For deltas, use $holidayType-&gt;field_activityid[1]-&gt;value; not providing it defaults to delta 0.

And, no, you definitely don't want an aggregate query. You need that if you want to COUNT() or SUM() some values of your entities.