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

db.collection.aggregate([{"$match":{"metadata.docId":{"$in":["7b96a"]}}},{"$unwind":"$items"},{"$project":{"metadata":1,"items.content":{$ifNull:["$items.contentWithInfo","$items.content"]}}}])