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db.collection.aggregate([{$set:{flattedItems:{$map:{input:"$items",in:{$objectToArray:"$$this"}}}}},{$set:{flattedItems:{$reduce:{input:"$flattedItems",initialValue:[],in:{$concatArrays:["$$value","$$this"]}}}}},{$set:{items:{$map:{input:{$setUnion:"$flattedItems.k"},in:{$arrayToObject:[[{k:"$$this",v:{$map:{input:{$filter:{input:"$flattedItems",as:"item",cond:{$eq:["$$this","$$item.k"]}}},in:"$$this.v"}}}]]}}}}},{$unset:"flattedItems"}])