可以根據條件來更新指定的文檔，但要注意查詢條件的唯一性：
> db.tianyc04.remove()
> db.tianyc04.insert({name:'tyc',age:20})
> db.tianyc04.insert({name:'tyc',age:30})
> db.tianyc04.insert({name:'tyc',age:32})
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 20 }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 30 }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
> x=db.tianyc04.findOne()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 20 }
> x.age++
20
> x
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 21 }
#按照name條件更新，將會更新所有記錄。但x中已經包含了_id字段，這樣會產生主鍵重復，所以mongo只會更新第一個。
> db.tianyc04.update({name:'tyc'}, x)
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 21 }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 30 }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
#如果我更新第二條數據，會怎么樣呢：
> x=db.tianyc04.findOne({age:30})
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 30 }
> x.age++
30
> x
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 31 }
> db.tianyc04.update({name:'tyc'},x)
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 21 }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 30 }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
> db.getLastError()
null
#可以看到第二條數據並沒有被更新掉。因為mongo會首先匹配到第一條記錄，嘗試將其更新為x，但此時會與第二條記錄產生主鍵重復，所以mongo會直接退出，且沒有提示錯誤。
#下面我按照_id來更新第二條記錄，這次就更新正常了：
> db.tianyc04.update({_id:ObjectId("510c83b113d7e50c2281b049")},x)
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 21 }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 31 }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
>
下面再無聊地使用tcy條件連續更新一下，就看到mongo的更新流程了：
> db.tianyc04.update({name:'tyc'},{name:'new_line'})
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "new_line" }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 31 }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
> db.tianyc04.update({name:'tyc'},{xjbu:'bengcaca'})
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "new_line" }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "xjbu" : "bengcaca" }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
> db.tianyc04.update({name:'tyc'},{tonight:'papapa'})
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "new_line" }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "xjbu" : "bengcaca" }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "tonight" : "papapa" }
>
總結：更新時的查詢條件建議選擇_id。