CRUD

在关系型数据库中，如果一个字段没有有效值，我们通常会初始化默认值（当然也可以存NULL），但在MongoDB中，我们可以不存储这个字段即可.这就是所谓的schema-less

Creating documents in mongodb

    - db.mycoll.insertOne(document, <optional params>) - db.mycoll.insertMany( [documents], <optional params> )

第二个参数如果是 “ordered”:false 则表示非顺序插入，其中某个document遇到异常，不影响其他document的插入。默认为True，顺序插入，会在错误处中断执行,则后续操作就不会执行了。

upsert （如果存在重复的先更新，否则直接插入。这种创建document的方式不详述）

每次插入document（一条数据），这个数据必须有一个“_id”字段，默认是自动生成一个唯一的ObjectId(…)，也可以手动指定一个唯一的数据。类似于传统数据库中的自增主键。
以上所说的“唯一”是在单个个collection中唯一，类似于传统数据库表的自增主键唯一，也只是这个table中唯一。
如下实例，可以看到“_id”除了自动生成，也可以手动设置（不建议手动设置，原因看后文）。

> db.elementMatch.insertMany([{result:[82,85,88]},{result:[75,88,89]}])
{"acknowledged" : true,"insertedIds" : [ObjectId("57558318c700c2881f75fef6"),ObjectId("57558318c700c2881f75fef7")]
}> db.moviesScratch.insertOne({ "title": "Rocky", "year": "1976", "imdb": "tt0075
148"});
{"acknowledged" : true,"insertedId" : ObjectId("5753c0eeeb62b409398b67ac")
}> db.moviesScratch.insertOne({ "title": "Rocky", "year": "1976", "imdb": "tt0075
", "_id":"10001"})
{ "acknowledged" : true, "insertedId" : "10001" }> db.moviesScratch.find({"title":"Rocky"}).pretty()
{"_id" : ObjectId("5753bcd0d72e989351cbf22a"),"title" : "Rocky","year" : "1976","imdb" : "tt0075148"
}
{"_id" : ObjectId("5753c0eeeb62b409398b67ac"),"title" : "Rocky","year" : "1976","imdb" : "tt0075148"
}
{ "_id" : "10001", "title" : "Rocky", "year" : "1976", "imdb" : "tt0075" }

详解_id字段

默认情况下，每个collection（表）都会在“_id”字段上有一个聚簇索引（可理解为主键索引）。并且这字段的默认类型是OjbectId.

{ "_id" : ObjectId("5753e1bfeb62b409398b67c1"), ...... }{ "_id" : ObjectId("5753e1e9eb62b409398b67c2"),......}

ObjetcId是一个12byte的16进制字符串组成如下：

Reading Documents

db.mycoll.find([query],[fields])
- query is an optional query filter. fields is optional set of fields to return.
  e.g.
  db.mycoll.find( {x:77} , {name:1, x:1} )
  ……………
  db.mycoll.find(…).count()
  db.mycoll.find(…).limit(n)
  db.mycoll.find(…).skip(n)
  db.mycoll.find(…).sort(…)
  db.mycoll.findOne([query], [fields], [options], [readConcern])

> db.movieDetails.find({"rated":"PG-13"}).count()
153
> db.movieDetails.find({"rated":"PG-13","year":2009}).count()
8

如何选择性读取内嵌fields

{｛....{"fieldOuterN": {...,"fieldInner": value}}....｝
}

如果要通过内嵌结构的字段查找,比如以下document中tomato字段中的meter等字段，使用如下方式 db.mycoll.find({“fieldOuterN.fieldInner” : value，……})：

> db.movieDetails.findOne()
{"_id" : ObjectId("569190ca24de1e0ce2dfcd4f"),"title" : "Once Upon a Time in the West","year" : 1968,"rated" : "PG-13","released" : ISODate("1968-12-21T05:00:00Z"),"runtime" : 175,"countries" : ["Italy","USA","Spain"],"genres" : ["Western"],
.........."tomato" : {"meter" : 98,"image" : "certified","rating" : 9,"reviews" : 54,"fresh" : 53,"consensus" : "A landmark Sergio Leone spaghetti western masterp
iece featuring a classic Morricone score.","userMeter" : 95,"userRating" : 4.3,"userReviews" : 64006},"metacritic" : 80,"awards" : {"wins" : 4,"nominations" : 5,"text" : "4 wins & 5 nominations."},"type" : "movie"
}> db.movieDetails.find({"tomato.rating":100}).count()
0
> db.movieDetails.find({"tomato.meter":100}).count()
8

如何选择性读取数组类型value

一般有以下几种读取场景：

过滤读取 整个数组
比如，读取 “writers” 字段值为 [“Ethan Coen”, “Joel Coen”]的document.(元素顺序、个数、内容等等完全匹配)

> db.movieDetails.find({"writers":["Ethan Coen","Joel Coen"]}).pretty()
{"_id" : ObjectId("5692a14f24de1e0ce2dfcf60"),"title" : "The Big Lebowski","year" : 1998,"rated" : "R","released" : ISODate("1998-03-06T05:00:00Z"),"runtime" : 117,"countries" : ["USA","UK"],"genres" : ["Comedy","Crime"],"director" : "Joel Coen, Ethan Coen","writers" : ["Ethan Coen","Joel Coen"],"actors" : ["Jeff Bridges","John Goodman","Julianne Moore","Steve Buscemi"],........................"type" : "movie"
}

过滤读取 数组中任一个元素
查找writers数组字段中含有 “Jeff Bridges”的所有document

> db.movieDetails.find({"actors":"Jeff Bridges"}).pretty()
{"_id" : ObjectId("569190d124de1e0ce2dfcd86"),"title" : "The Last Picture Show","year" : 1971,"rated" : "R","released" : ISODate("1971-10-22T04:00:00Z"),"runtime" : 118,"countries" : ["USA"],"genres" : ["Drama"],"director" : "Peter Bogdanovich","writers" : ["Larry McMurtry","Peter Bogdanovich","Larry McMurtry"],"actors" : ["Timothy Bottoms","Jeff Bridges","Cybill Shepherd","Ben Johnson"],"plot" : "A group of 1950s high schoolers come of age in a bleak, isolat
ed, atrophied West Texas town that is slowly dying, both economically and cultur
ally.","poster" : "http://ia.media-imdb.com/images/M/MV5BNDkyNzQ1NzYzN15BMl5Ban
BnXkFtZTcwNjE5MDEwNw@@._V1._CR73.19999694824219,89.69999694824219,1196,1865.2000
274658203_SX89_AL_.jpg_V1_SX300.jpg","imdb" : {"id" : "tt0067328","rating" : 8.1,"votes" : 29830},"awards" : {"wins" : 16,"nominations" : 22,"text" : "Won 2 Oscars. Another 16 wins & 22 nominations."},"type" : "movie"
}
..............................
{"_id" : ObjectId("5692a14e24de1e0ce2dfcf56"),"title" : "Crazy Heart","year" : 2009,"rated" : "R","released" : ISODate("2010-02-05T05:00:00Z"),"runtime" : 112,"countries" : ["USA"],"genres" : ["Drama","Music","Romance"],"director" : "Scott Cooper","writers" : ["Scott Cooper","Thomas Cobb"],"actors" : ["Jeff Bridges","James Keane","Anna Felix","Paul Herman"],"plot" : "A faded country music musician is forced to reassess his dysfu
nctional life during a doomed romance that also inspires him.","poster" : "http://ia.media-imdb.com/images/M/MV5BMTU0NDc5NjgzNl5BMl5Ban
BnXkFtZTcwNzc0NDIzMw@@._V1_SX300.jpg","imdb" : {"id" : "tt1263670","rating" : 7.3,"votes" : 63771},"tomato" : {"meter" : 91,"image" : "certified","rating" : 7.4,"reviews" : 199,"fresh" : 181,"consensus" : "Thanks to a captivating performance from Jeff Bri
dges, Crazy Heart transcends its overly familiar origins and finds new meaning i
n an old story.","userMeter" : 76,"userRating" : 3.6,"userReviews" : 124175},"metacritic" : 83,"awards" : {"wins" : 32,"nominations" : 26,"text" : "Won 2 Oscars. Another 32 wins & 26 nominations."},"type" : "movie"
}>

过滤读取 数组中指定的某位置的者某特定元素
比如我要查找actors字段的第三个元素为“Jeff Bridges”的document。

find({"actors.2":"Jeff Bridges"})

> db.movieDetails.find({"actors.2":"Jeff Bridges"}).pretty()
{"_id" : ObjectId("5692a15424de1e0ce2dfcf98"),"title" : "Iron Man","year" : 2008,"rated" : "PG-13","released" : ISODate("2008-05-02T04:00:00Z"),"runtime" : 126,"countries" : ["USA"],"genres" : ["Action","Adventure","Sci-Fi"],"director" : "Jon Favreau","writers" : ["Mark Fergus","Hawk Ostby","Art Marcum","Matt Holloway","Stan Lee","Don Heck","Larry Lieber","Jack Kirby"],"actors" : ["Robert Downey Jr.","Terrence Howard","Jeff Bridges","Gwyneth Paltrow"],"plot" : "After being held captive in an Afghan cave, an industrialist c
reates a unique weaponized suit of armor to fight evil.","poster" : "http://ia.media-imdb.com/images/M/MV5BMTczNTI2ODUwOF5BMl5Ban
BnXkFtZTcwMTU0NTIzMw@@._V1_SX300.jpg","imdb" : {"id" : "tt0371746","rating" : 7.9,"votes" : 641369},"tomato" : {"meter" : 94,"image" : "certified","rating" : 7.7,"reviews" : 266,"fresh" : 249,"consensus" : "Director Jon Favreau and star Robert Downey Jr. m
ake this smart, high impact superhero movie one that even non-comics fans can en
joy.","userMeter" : 91,"userRating" : 4.2,"userReviews" : 1072887},"metacritic" : 79,"awards" : {"wins" : 19,"nominations" : 58,"text" : "Nominated for 2 Oscars. Another 19 wins & 58 nominatio
ns."},"type" : "movie"
}
>

更复杂的匹配读取，后文中再讨论

CURSOR指针

前文说过，在shell中，db.mycoll.find()返回是一个指针对象。而且默认情况下，每次返回内容默认是这个指针对象遍历20次的内容集合。

.......
Type "it" for more

通过使用it再次遍历20次，并返回20次的总结果集合。

通常，mongoDB是以batches（批量）方式返回查询集合。对于大多数查询，第一个批量返回101条document.比如:
第一个批量总共返回101条document，第二个批量返回剩余的21条document。

> db.funnynumbers.find().count()
122> var iterator=db.funnynumbers.find();
> var eachDoc=function(){return iterator.hasNext()?iterator.next():null}
> iterator.objsLeftInBatch()
101
> eachDoc()
{ "_id" : ObjectId("50778ce69331a280cf4bcf7d"), "value" : 87 }
> iterator.objsLeftInBatch()
100
> eachDoc()
{ "_id" : ObjectId("50778ce69331a280cf4bcf7e"), "value" : 34 }
> iterator.objsLeftInBatch()
99

Projection

一种实现部分缩减返回数据的手段（通常针对单个查询）。通过减少返回document中的fields,来减少网络开销以及系统负载。
通常 Projection通过设置db.mycoll.find({},{})的第二个参数来实现。

比如：我要只返回（过滤：tomato字段中的meter字段为100的）document中的awards字段（可以看到“_id”并没有限制住，默认返回）。

> db.movieDetails.findOne({"tomato.meter":100},{"awards":1})
{"_id" : ObjectId("569190d024de1e0ce2dfcd79"),"awards" : {"wins" : 2,"nominations" : 2,"text" : "Won 3 Oscars. Another 2 wins & 2 nominations."}
}
>
>
> db.movieDetails.findOne({"tomato.meter":100},{type:0,tomato:0,writers:0,actors
:0,"_id":0,countries:0,genres:0})
{"title" : "The Adventures of Robin Hood","year" : 1938,"rated" : "PG","released" : ISODate("1938-05-14T04:00:00Z"),"runtime" : 102,"director" : "Michael Curtiz, William Keighley","plot" : "When Prince John and the Norman Lords begin oppressing the Sax
on masses in King Richard's absence, a Saxon lord fights back as the outlaw lead
er of a rebel guerrilla army.","poster" : "http://ia.media-imdb.com/images/M/MV5BMTUxMzE0MjI0MF5BMl5Ban
BnXkFtZTcwMTQ1OTM2MQ@@._V1_SX300.jpg","imdb" : {"id" : "tt0029843","rating" : 8,"votes" : 37543},"metacritic" : 97,"awards" : {"wins" : 2,"nominations" : 2,"text" : "Won 3 Oscars. Another 2 wins & 2 nominations."}
}
>
>> db.movieDetails.findOne({"tomato.meter":100},{"awards":1,_id:0})
{"awards" : {"wins" : 2,"nominations" : 2,"text" : "Won 3 Oscars. Another 2 wins & 2 nominations."}
}

所以:
find（）第二参数：｛“awards”:1,”_id”:0｝：
1表示只有这个字段需要返回（“_id”默认返回）；
0表示返回除这个字段之外的其他字段。

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