与更新现有集合字段相同，$set 如果指定的字段不存在，则会添加一个新的字段.

Same as the updating existing collection field, $set will add a new fields if the specified field does not exist.

看看这个例子:

> db.foo.find()
> db.foo.insert({"test":"a"})
> db.foo.find()
{ "_id" : ObjectId("4e93037bbf6f1dd3a0a9541a"), "test" : "a" }
> item = db.foo.findOne()
{ "_id" : ObjectId("4e93037bbf6f1dd3a0a9541a"), "test" : "a" }
> db.foo.update({"_id" :ObjectId("4e93037bbf6f1dd3a0a9541a") },{$set : {"new_field":1}})
> db.foo.find()
{ "_id" : ObjectId("4e93037bbf6f1dd3a0a9541a"), "new_field" : 1, "test" : "a" }

如果你想在你的所有集合中添加一个 new_field，你必须使用空选择器，并将 multi 标志设置为 true(最后一个参数)以更新所有文档

In case you want to add a new_field to all your collection, you have to use empty selector, and set multi flag to true (last param) to update all the documents

db.your_collection.update(
  {},
  { $set: {"new_field": 1} },
  false,
  true
)

在上面的例子中，最后 2 个字段 false, true 指定了 upsert 和 multi 标志.

In the above example last 2 fields false, true specifies the upsert and multi flags.

Upsert: 如果设置为 true，则在没有文档与查询条件匹配时创建一个新文档.

Upsert: If set to true, creates a new document when no document matches the query criteria.

Multi: 如果设置为 true，则更新满足查询条件的多个文档.如果设置为 false，则更新一个文档.

Multi: If set to true, updates multiple documents that meet the query criteria. If set to false, updates one document.

这适用于 2.2 之前的 Mongo versions.对于最新版本，查询略有变化

This is for Mongo versions prior to 2.2. For latest versions the query is changed a bit

db.your_collection.update({},
                          {$set : {"new_field":1}},
                          {upsert:false,
                          multi:true})