往常使用options.Find().SetSort(bson.D{{"a", -1},{"b", -1}})结果报错，感觉使用方法没问题，排查之后发现是import的包错了导致。
cannot transform type bson.D to a BSON Document: WriteArray can only write a Array while positioned on a Element or Value but is positioned on a TopLevel
"gopkg.in/mgo.v2/bson"换成"go.mongodb.org/mongo-driver/bson"就没问题了。