Mongodb用户手册-英文版
[*]Getting the Database
[*]Getting A Database Connection
[*]Inserting Data into A Collection
[*]Accessing Data From a Query
[*]Specifying What the Query Returns
[*] findOne() - Syntactic Sugar
[*]Limiting the Result Set via limit()
[*]More Help
[*]What Next
Getting the Database
First, run through the Quickstart guide for your platform to get up and running.
Getting A Database Connection
Let's now try manipulating the database with the database shell . (We could perform similar operations from any programming language using an appropriate driver.The shell is convenient for interactive and administrative use.)
Start the MongoDB JavaScript shell with:
# 'mongo' is shell binary. exact location might vary depending on
# installation method and platform
$ bin/mongo
By default the shell connects to database "test" on localhost.You then see:
MongoDB shell version:
url: test
connecting to: test
type "help" for help
>
"connecting to:" tells you the name of the database the shell is using.To switch databases, type:
> use mydb
switched to db mydb
To see a list of handy commands, type help.
http://www.mongodb.org/images/icons/emoticons/information.gif
Tip for Developers with Experience in Other Databases
Youmay notice, in the examples below, that we never create a database orcollection.MongoDB does not require that you do so.As soon as youinsert something, MongoDB creates the underlying collection anddatabase.If you query a collection that does not exist, MongoDB treatsit as an empty collection.
Switching to a database with the use command won'timmediately create the database - the database is created lazily thefirst time data is inserted. This means that if you use a database for the first time it won't show up in the list provided by `show dbs` until data is inserted.
Inserting Data into A Collection
Let's create a test collection and insert some data into it.We will create two objects, j and t, and then save them in the collection things.
In the following examples, '>' indicates commands typed at the shell prompt.
> j = { name : "mongo" };
{"name" : "mongo"}
> t = { x : 3 };
{ "x" : 3}
> db.things.save(j);
> db.things.save(t);
> db.things.find();
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
{ "_id" : ObjectId("4c2209fef3924d31102bd84b"), "x" : 3 }
>
A few things to note :
[*]We did not predefine the collection.The database creates it automatically on the first insert.
[*]The documents we store can have any "structure" - in fact in thisexample, the documents have no common data elements at all.Inpractice, one usually stores documents of the same structure withincollections.However, this flexibility means that schema migration andaugmentation are very easy in practice - rarely will you need to writescripts which perform "alter table" type operations.
[*]Upon being inserted into the database, objects are assigned an object>
[*]When you run the above example, your ObjectID values will be different.
Let's add some more records to this collection:
> for (var i = 1; idb.things.find();
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
{ "_id" : ObjectId("4c2209fef3924d31102bd84b"), "x" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "x" : 4, "j" : 1 }
{ "_id" : ObjectId("4c220a42f3924d31102bd857"), "x" : 4, "j" : 2 }
{ "_id" : ObjectId("4c220a42f3924d31102bd858"), "x" : 4, "j" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd859"), "x" : 4, "j" : 4 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85a"), "x" : 4, "j" : 5 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85b"), "x" : 4, "j" : 6 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85c"), "x" : 4, "j" : 7 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85d"), "x" : 4, "j" : 8 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85e"), "x" : 4, "j" : 9 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85f"), "x" : 4, "j" : 10 }
{ "_id" : ObjectId("4c220a42f3924d31102bd860"), "x" : 4, "j" : 11 }
{ "_id" : ObjectId("4c220a42f3924d31102bd861"), "x" : 4, "j" : 12 }
{ "_id" : ObjectId("4c220a42f3924d31102bd862"), "x" : 4, "j" : 13 }
{ "_id" : ObjectId("4c220a42f3924d31102bd863"), "x" : 4, "j" : 14 }
{ "_id" : ObjectId("4c220a42f3924d31102bd864"), "x" : 4, "j" : 15 }
{ "_id" : ObjectId("4c220a42f3924d31102bd865"), "x" : 4, "j" : 16 }
{ "_id" : ObjectId("4c220a42f3924d31102bd866"), "x" : 4, "j" : 17 }
{ "_id" : ObjectId("4c220a42f3924d31102bd867"), "x" : 4, "j" : 18 }
has more
Note that not all documents were shown - the shell limits the numberto 20 when automatically iterating a cursor.Since we already had 2documents in the collection, we only see the first 18 of thenewly-inserted documents.
If we want to return the next set of results, there's the it shortcut. Continuing from the code above:
{ "_id" : ObjectId("4c220a42f3924d31102bd866"), "x" : 4, "j" : 17 }
{ "_id" : ObjectId("4c220a42f3924d31102bd867"), "x" : 4, "j" : 18 }
has more
> it
{ "_id" : ObjectId("4c220a42f3924d31102bd868"), "x" : 4, "j" : 19 }
{ "_id" : ObjectId("4c220a42f3924d31102bd869"), "x" : 4, "j" : 20 }
Technically, find() returns a cursor object. But in the cases above,we haven't assigned that cursor to a variable. So, the shellautomatically iterates over the cursor, giving us an initial result set,and allowing us to continue iterating with the it command.
But we can also work with the cursor directly; just how that's done is discussed in the next section.
Accessing Data From a Query
Before we discuss queries in any depth, lets talk about how to workwith the results of a query - a cursor object.We'll use the simple find() query method, which returns everything in a collection, and talk about how to create specific queries later on.
In order to see all the elements in the collection when using the mongo shell, we need to explicitly use the cursor returned from the find() operation.
Lets repeat the same query, but this time use the cursor that find() returns, and iterate over it in a while loop :
> var cursor = db.things.find();
> while (cursor.hasNext()) printjson(cursor.next());
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
{ "_id" : ObjectId("4c2209fef3924d31102bd84b"), "x" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "x" : 4, "j" : 1 }
{ "_id" : ObjectId("4c220a42f3924d31102bd857"), "x" : 4, "j" : 2 }
{ "_id" : ObjectId("4c220a42f3924d31102bd858"), "x" : 4, "j" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd859"), "x" : 4, "j" : 4 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85a"), "x" : 4, "j" : 5 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85b"), "x" : 4, "j" : 6 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85c"), "x" : 4, "j" : 7 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85d"), "x" : 4, "j" : 8 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85e"), "x" : 4, "j" : 9 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85f"), "x" : 4, "j" : 10 }
{ "_id" : ObjectId("4c220a42f3924d31102bd860"), "x" : 4, "j" : 11 }
{ "_id" : ObjectId("4c220a42f3924d31102bd861"), "x" : 4, "j" : 12 }
{ "_id" : ObjectId("4c220a42f3924d31102bd862"), "x" : 4, "j" : 13 }
{ "_id" : ObjectId("4c220a42f3924d31102bd863"), "x" : 4, "j" : 14 }
{ "_id" : ObjectId("4c220a42f3924d31102bd864"), "x" : 4, "j" : 15 }
{ "_id" : ObjectId("4c220a42f3924d31102bd865"), "x" : 4, "j" : 16 }
{ "_id" : ObjectId("4c220a42f3924d31102bd866"), "x" : 4, "j" : 17 }
{ "_id" : ObjectId("4c220a42f3924d31102bd867"), "x" : 4, "j" : 18 }
{ "_id" : ObjectId("4c220a42f3924d31102bd868"), "x" : 4, "j" : 19 }
{ "_id" : ObjectId("4c220a42f3924d31102bd869"), "x" : 4, "j" : 20 }
The above example shows cursor-style iteration. The hasNext() function tells if there are any more documents to return, and the next() function returns the next document.We also used the built-in tojson() method to render the document in a pretty JSON-style format.
When working in the JavaScript shell, we can also use the functional features of the language, and just call forEach on the cursor.Repeating the example above, but using forEach() directly on the cursor rather than the while loop:
> db.things.find().forEach(printjson);
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
{ "_id" : ObjectId("4c2209fef3924d31102bd84b"), "x" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "x" : 4, "j" : 1 }
{ "_id" : ObjectId("4c220a42f3924d31102bd857"), "x" : 4, "j" : 2 }
{ "_id" : ObjectId("4c220a42f3924d31102bd858"), "x" : 4, "j" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd859"), "x" : 4, "j" : 4 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85a"), "x" : 4, "j" : 5 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85b"), "x" : 4, "j" : 6 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85c"), "x" : 4, "j" : 7 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85d"), "x" : 4, "j" : 8 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85e"), "x" : 4, "j" : 9 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85f"), "x" : 4, "j" : 10 }
{ "_id" : ObjectId("4c220a42f3924d31102bd860"), "x" : 4, "j" : 11 }
{ "_id" : ObjectId("4c220a42f3924d31102bd861"), "x" : 4, "j" : 12 }
{ "_id" : ObjectId("4c220a42f3924d31102bd862"), "x" : 4, "j" : 13 }
{ "_id" : ObjectId("4c220a42f3924d31102bd863"), "x" : 4, "j" : 14 }
{ "_id" : ObjectId("4c220a42f3924d31102bd864"), "x" : 4, "j" : 15 }
{ "_id" : ObjectId("4c220a42f3924d31102bd865"), "x" : 4, "j" : 16 }
{ "_id" : ObjectId("4c220a42f3924d31102bd866"), "x" : 4, "j" : 17 }
{ "_id" : ObjectId("4c220a42f3924d31102bd867"), "x" : 4, "j" : 18 }
{ "_id" : ObjectId("4c220a42f3924d31102bd868"), "x" : 4, "j" : 19 }
{ "_id" : ObjectId("4c220a42f3924d31102bd869"), "x" : 4, "j" : 20 }
In the case of a forEach() we must define a function that is called for each document in the cursor.
In the mongo shell, you can also treat cursors like an array :
> var cursor = db.things.find();
> printjson(cursor);
{ "_id" : ObjectId("4c220a42f3924d31102bd858"), "x" : 4, "j" : 3 }
When using a cursor this way, note that all values up to the highestaccessed (cursor above) are loaded into RAM at the same time.Thisis inappropriate for large result sets, as you will run out of memory. Cursors should be used as an iterator with any query which returns alarge number of elements.
In addition to array-style access to a cursor, you may also convert the cursor to a true array:
> var arr = db.things.find().toArray();
> arr;
{ "_id" : ObjectId("4c220a42f3924d31102bd859"), "x" : 4, "j" : 4 }
Please note that these array features are specific to mongo - The Interactive Shell, and not offered by all drivers.
MongoDB cursors are not snapshots - operations performed by you orother users on the collection being queried between the first and lastcall to next() of your cursor may or may not be returned by the cursor.Use explicit locking to perform a snapshotted query.
Specifying What the Query Returns
Now that we know how to work with the cursor objects that arereturned from queries, lets now focus on how to tailor queries to returnspecific things.
In general, the way to do this is to create "query documents", whichare documents that indicate the pattern of keys and values that are tobe matched.
These are easier to demonstrate than explain.In the followingexamples, we'll give example SQL queries, and demonstrate how torepresent the same query using MongoDB via the mongo shell.This way of specifying queries is fundamental to MongoDB, so you'll find the same general facility in any driver or language.
SELECT * FROM things WHERE name="mongo"
> db.things.find({name:"mongo"}).forEach(printjson);
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
SELECT * FROM things WHERE x=4
> db.things.find({x:4}).forEach(printjson);
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "x" : 4, "j" : 1 }
{ "_id" : ObjectId("4c220a42f3924d31102bd857"), "x" : 4, "j" : 2 }
{ "_id" : ObjectId("4c220a42f3924d31102bd858"), "x" : 4, "j" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd859"), "x" : 4, "j" : 4 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85a"), "x" : 4, "j" : 5 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85b"), "x" : 4, "j" : 6 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85c"), "x" : 4, "j" : 7 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85d"), "x" : 4, "j" : 8 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85e"), "x" : 4, "j" : 9 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85f"), "x" : 4, "j" : 10 }
{ "_id" : ObjectId("4c220a42f3924d31102bd860"), "x" : 4, "j" : 11 }
{ "_id" : ObjectId("4c220a42f3924d31102bd861"), "x" : 4, "j" : 12 }
{ "_id" : ObjectId("4c220a42f3924d31102bd862"), "x" : 4, "j" : 13 }
{ "_id" : ObjectId("4c220a42f3924d31102bd863"), "x" : 4, "j" : 14 }
{ "_id" : ObjectId("4c220a42f3924d31102bd864"), "x" : 4, "j" : 15 }
{ "_id" : ObjectId("4c220a42f3924d31102bd865"), "x" : 4, "j" : 16 }
{ "_id" : ObjectId("4c220a42f3924d31102bd866"), "x" : 4, "j" : 17 }
{ "_id" : ObjectId("4c220a42f3924d31102bd867"), "x" : 4, "j" : 18 }
{ "_id" : ObjectId("4c220a42f3924d31102bd868"), "x" : 4, "j" : 19 }
{ "_id" : ObjectId("4c220a42f3924d31102bd869"), "x" : 4, "j" : 20 }
The query expression is an document itself.A query document of theform { a:A, b:B, ... } means "where a==A and b==B and ...".Moreinformation on query capabilities may be found in the Queries and Cursors section of the Mongo Developers' Guide.
MongoDB also lets you return "partial documents" - documents thathave only a subset of the elements of the document stored in thedatabase.To do this, you add a second argument to the find() query, supplying a document that lists the elements to be returned.
To illustrate, lets repeat the last example find({x:4}) with an additional argument that limits the returned document to just the "j" elements:
SELECT j FROM things WHERE x=4
> db.things.find({x:4}, {j:true}).forEach(printjson);
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "j" : 1 }
{ "_id" : ObjectId("4c220a42f3924d31102bd857"), "j" : 2 }
{ "_id" : ObjectId("4c220a42f3924d31102bd858"), "j" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd859"), "j" : 4 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85a"), "j" : 5 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85b"), "j" : 6 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85c"), "j" : 7 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85d"), "j" : 8 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85e"), "j" : 9 }
{ "_id" : ObjectId("4c220a42f3924d31102bd85f"), "j" : 10 }
{ "_id" : ObjectId("4c220a42f3924d31102bd860"), "j" : 11 }
{ "_id" : ObjectId("4c220a42f3924d31102bd861"), "j" : 12 }
{ "_id" : ObjectId("4c220a42f3924d31102bd862"), "j" : 13 }
{ "_id" : ObjectId("4c220a42f3924d31102bd863"), "j" : 14 }
{ "_id" : ObjectId("4c220a42f3924d31102bd864"), "j" : 15 }
{ "_id" : ObjectId("4c220a42f3924d31102bd865"), "j" : 16 }
{ "_id" : ObjectId("4c220a42f3924d31102bd866"), "j" : 17 }
{ "_id" : ObjectId("4c220a42f3924d31102bd867"), "j" : 18 }
{ "_id" : ObjectId("4c220a42f3924d31102bd868"), "j" : 19 }
{ "_id" : ObjectId("4c220a42f3924d31102bd869"), "j" : 20 }
Note that the "_id" field is always returned.
findOne() - Syntactic Sugar
For convenience, the mongo shell(and other drivers) lets you avoid the programming overhead of dealingwith the cursor, and just lets you retrieve one document via the findOne() function. findOne() takes all the same parameters of the find() function, but instead of returning a cursor, it will return either the first document returned from the database, or null if no document is found that matches the specified query.
As an example, lets retrieve the one document with name=='mongo'.There are many ways to do it, including just calling next() on the cursor (after checking for null, of course), or treating the cursor as an array and accessing the 0th element.
However, the findOne() method is both convenient and efficient:
> printjson(db.things.findOne({name:"mongo"}));
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
This is more efficient because the client requests a single objectfrom the database, so less work is done by the database and the network. This is the equivalent of find({name:"mongo"}).limit(1).
Limiting the Result Set via limit()
You may limit the> This is highly recommended for performance reasons, as it limits thework the database does, and limits the amount of data returned over thenetwork.For example:
> db.things.find().limit(3);
{ "_id" : ObjectId("4c2209f9f3924d31102bd84a"), "name" : "mongo" }
{ "_id" : ObjectId("4c2209fef3924d31102bd84b"), "x" : 3 }
{ "_id" : ObjectId("4c220a42f3924d31102bd856"), "x" : 4, "j" : 1 }
More Help
In addition to the general "help" command, you can call help on db and db.whatever to see a summary of methods available.
If you are curious about what a function is doing, you can type itwithout the {{()}}s and the shell will print the source, for example:
> printjson
function (x) {
print(tojson(x));
}
mongo is a full JavaScript shell, so any JavaScriptfunction, syntax, or>
What Next
[*]After completing this tutorial the next step to learning MongoDB is to dive into the manual for more details.
[*]See also SQL to Mongo Mapping Chart
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