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forumBoardGroup := bson.D{{"$lookup", bson.M{
   "from": "forum_sub_board",
   "let":  bson.M{"boardId": "$_id"},
   "pipeline": mongo.Pipeline{
      bson.D{{"$match", bson.M{"$expr": bson.A{bson.M{"$eq": bson.A{"$board_id", "$$boardId"}}}}}},

   },
   "as": "sub_board",
}}}

board_id和Id都是objectId类型,但是就是没法匹配,感觉是这里的match根本没起作用,求教,有朋友遇到过吗?求教下怎么解决。
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