code: public static Bson elemMatch(final String fieldName, final Bson filter) {
          return new Bson() {
              @Override
              public <TDocument> BsonDocument toBsonDocument(final Class<TDocument> documentClass, final CodecRegistry codecRegistry) {
                  return new BsonDocument(fieldName, new BsonDocument("$elemMatch", filter.toBsonDocument(documentClass, codecRegistry)));
              }
          };
      }
docs: Creates a filter that matches all documents containing a field that is an array where at least one member of the array matches the
  given filter.
  
  @param fieldName the field name
  @param filter    the filter to apply to each element
  @return the filter
  @mongodb.driver.manual reference/operator/query/elemMatch $elemMatch