db.articles.aggregate([
  {"$unwind": "$tags"},
  {
    "$lookup": {
      "localField": "tags",
      "from": "tags",
      "foreignField": "_id",
      "as": "materialTags"
    }
 },
 {
    "$group": {
      "_id": "$_id",
      "title": {"$first": "$title"},
      "materialTags": {"$push": "$materialTags"}
    }
  }
])

UnwindOperation unwindOperation = Aggregation.unwind("tags");
LookupOperation lookupOperation1 = LookupOperation.newLookup()
    .from("tags")
    .localField("tags")
    .foreignField("_id")
    .as("materialTags");

//I also want to add group operation but unable to find the proper syntax ??.
Aggregation aggregation = Aggregation.newAggregation(unwindOperation,
 lookupOperation1, ??groupOperation?? );
AggregationResults<Article> resultList
    = mongoTemplate.aggregate(aggregation, "articles", Article.class);