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新手上路,Mongodb update操作更新二維數組有趣的現象各位有什么建議？

JavaScript

慕虎7371278 2019-08-21 19:39:47

問題描述有如下數據:{"_id":1,"name":"dave123","favorites":["chocolate","cake","butter","apples"]}{"_id":2,"name":"li","favorites":["apples","pudding","pie","cake"]}{"_id":3,"name":"ahn","favorites":["pears","pecans","chocolate","cherries"]}{"_id":4,"name":"ty","favorites":["icecream"]}然后我剛才操作失誤,原本想給數組中添加一個元素結果給添加了一個數組進去:db.test.update({_id:2},{$push:{favorites:['cake']}})執(zhí)行后的數據如下:{"_id":1,"name":"dave123","favorites":["chocolate","cake","butter","apples"]}{"_id":2,"name":"li","favorites":["apples","pudding","pie","cake",["cake"]]}{"_id":3,"name":"ahn","favorites":["pears","pecans","chocolate","cherries"]}可以看到id2的文檔的favorites字段中又包含了一個數組想法設法刪除掉這個數組我知道$pull命令可以刪除掉數組中的一個元素,但是它需要一個條件,于是問題被引到了如何在數組中查詢另外一個數組.命令如下:db.tester.find({favorites:{$in:[['cake']]}})輸出:{"_id":2,"name":"li","favorites":["apples","pudding","pie","cake",["cake"]]}然后我嘗試使用update來完成這個操作,由于集合中就這么一條特殊數據我就沒有指定條件:db.tester.update({},{$pull:{favorites:{$in:[['cake']]}}})輸出:WriteResult({"nMatched":1,"nUpserted":0,"nModified":0})喜聞樂見,匹配到了東西但是沒有更新東西.繼續(xù)折騰在我的努力下試出來了這么一個命令:db.tester.update({favorites:{$in:[['cake']]}},{$pull:{favorites:{$in:[['cake']]}}})輸出:WriteResult({"nMatched":1,"nUpserted":0,"nModified":1})這次這個數組中的元素被正確的刪除掉了.最終的問題WHY?udpate操作中查詢和跟隨著的修改操作有什么潛在的關系嗎,還是和$pull中的條件有關系?

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寶慕林4294392

TA貢獻2021條經驗獲得超8個贊

答案就像我猜的一樣,是一個細節(jié)問題.Bydefault,theupdate()methodupdatesasingledocument.默認情況下update只會修改匹配到的第一條數據.在我使用的語句中:db.tester.update({},{$pull:{favorites:{$in:[['cake']]}}})這條命令會匹配多條數據,鑒于上面提到的規(guī)則MongoDB匹配到了多個數據后實際交由$pull操作的只有第一條:{"_id":1,"name":"dave123","favorites":["chocolate","cake","butter","apples"]}也就是上面這條,但是$pull條件卻不成立了,所以不會進行任何更新.這也解釋了為什么如下的操作可行:db.test.update({favorites:{$in:[['cake']]}},{$pull:{favorites:{$in:[['cake']]}}})查詢只會匹配到第一條數據而這條數據則會交由$pull處理.如何解決問題很簡單在這個情況下開啟多條修改支持就可以了,命令如下:db.tester.update({},{$pull:{favorites:{$in:[['cake']]}}},{multi:true})這樣一來查詢到的四條數據都會將由后面的更新處理而$pull也會匹配到對應的內容.我是如何找到問題的觀察我們的前兩條數據:{"_id":1,"name":"dave123","favorites":["chocolate","cake","butter","apples"]}{"_id":2,"name":"li","favorites":["apples","pudding","pie","cake"]}我嘗試刪除兩者數組中都存在的cake元素:db.test.update({},{$pull:{favorites:{$in:['cake']}}})但是只會刪除第一條,第二條數據會進行匹配但是不會刪除.

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蕭十郎

TA貢獻1815條經驗獲得超13個贊

rs0:PRIMARY>db.test.insert({"_id":2,"name":"li","favorites":["apples","pudding","pie","cake",["cake"]]})rs0:PRIMARY>db.test.update({},{$pull:{favorites:{$in:[['cake']]}}})WriteResult({"nMatched":1,"nUpserted":0,"nModified":1})rs0:PRIMARY>db.test.find(){"_id":2,"name":"li","favorites":["apples","pudding","pie","cake"]}看上去我并不能重現這個問題。你的版本是？

反對回復 2019-08-21