[Question/Support] Monoid Aggregation support

[KinWang-2013]

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I am using mongoid aggregation which finally returns an array(i do to_a).
example: Model.collection.aggregate(pipeline).to_a

In my index API, I use blueprinter with association:
example:
association :creator, blueprint: UserBlueprint

but when I do render_as_json the creator is null

am I doing something wrong?