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db.collection.aggregate([{$set:{items:"$$REMOVE",users:{$map:{input:"$users",in:{$mergeObjects:["$$this",{items:"$items"}]}}}}},{$unwind:"$users"},{$group:{_id:"$users.name",orders:{$addToSet:"$orderid"},unique_items:{$addToSet:"$users.items.name"}}},{$set:{unique_items:{$reduce:{input:"$unique_items",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}}},{$set:{unique_items:{$size:{$setUnion:"$unique_items"}}}}])