db={orders:[{"id":"1","items":{"itemId":"1000","name":"Item 1","status":"STATUS"}},{"id":"2","items":{"itemId":"1000","name":"Item 1","status":"ANOTHER_STATUS"}}],item_projections:[{"id":"1","itemId":"1000","name":"Item 1","orderId":"1"},{"id":"1","itemId":"1000","name":"Item 1","orderId":"2"}]}

db.item_projections.aggregate([{"$lookup":{"from":"orders","localField":"orderId","foreignField":"id","pipeline":[{$project:{status:"$items.status"}}],"as":"orders"}},{$replaceRoot:{newRoot:{$mergeObjects:["$$ROOT",{$first:"$orders"}]}}},{$unset:"orders"}])