[{"metadata":{"docId":"7b96a"},"items":{"content":"abcd",/**"contentWithInfo": "content with additional info"*/}}]

db.collection.aggregate([{"$match":{"metadata.docId":{"$in":["7b96a"]}}},{"$unwind":"$items"},{"$project":{"metadata":1,"items.content":{"$cond":{"if":{"$eq":[{$type:"$items.contentWithInfo"},"missing"]},"then":"$items.content","else":"$items.contentWithInfo"}/**$ifNull: [* "$items.contentWithInfo",* "$items.content"*]*/}}}])