$ concat是聚合管道，而不是更新运算符/修饰符。

$concat is an aggregation pipeline, not an update operator/modifier.

似乎您可以通过以下操作来实现：

It seems that what you're trying to do can be achieved by doing the following:

db.col_1
  .find({ "field_1": { $lt: 10000 } })
  .forEach( function(i) {
    db.col_1.update(
      { _id: i._id },
      { $set: { "field_1": "0" + i.field_1 } }
    )
   });