MongoDB 3.2.4官方文档：全面指南与关键特性

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本文档是MongoDB 3.2.4版本的官方说明文档，由MongoDB, Inc.发布于2016年4月6日。MongoDB是一款流行的分布式文档型数据库，它提供了灵活的数据模型、强大的查询能力以及易于扩展的特性。以下是文档的主要内容概览： 1. **MongoDB简介**： - MongoDB是一种基于键值对的文档数据库，它以JSON风格存储数据，支持丰富的数据结构和嵌套，非常适合处理半结构化或非结构化的数据。 - 该版本的重要特性包括：强大的数据模型支持、高性能的查询性能、自动分片和复制集等，以满足大规模数据存储和处理的需求。 2. **安装MongoDB**： - 文档介绍了在不同平台（如Windows、Linux、macOS）上安装MongoDB的方法，同时提及了32位版本的停用，并推荐用户升级到64位版本。 - 提供了安装教程以及额外资源链接，帮助用户顺利安装和配置环境。 3. **MongoDB CRUD操作**： - CRUD（Create, Read, Update, Delete）操作是数据库操作的核心。文档详细讲解了如何执行基本操作、概念和实践教程，以及提供了一份参考手册，便于开发者进行高效的数据管理。 4. **数据模型设计**： - 数据模型设计是MongoDB中的关键环节。文档涵盖了数据建模的入门、验证、概念和实例，还提供了最佳实践和详细参考，帮助用户设计适应业务需求的高效数据架构。 5. **数据库管理和运维**： - 这部分包括数据库的日常维护和监控，如管理概念、教程，以及更深入的参考材料。生产检查清单确保了在生产环境中进行有效的管理和优化。 6. **安全性**： - 安全性是现代数据库的重要考量。文档列出了安全检查清单，涉及认证、角色基础访问控制、数据加密、审计等方面，以保护敏感信息和系统安全。总结来说，MongoDB 3.2.4版本的文档详细覆盖了从数据库基础知识、安装和配置到高级操作和安全性的全方位内容，是开发人员和管理员理解和使用MongoDB的强大指南。无论是初次接触还是经验丰富的用户，都能从中找到所需的信息和支持。

MongoDB Documentation, Release 3.2.4

Changed in version 2.6: Starting in version 2.6, MongoDB actively attempts to preserve the ﬁeld order in a document.

Before version 2.6, MongoDB did not actively preserve the order of the ﬁelds in a document.

The _id Field The _id ﬁeld has the following behavior and constraints:

• By default, MongoDB creates a unique index on the _id ﬁeld during the creation of a collection.

• The _id ﬁeld is always the ﬁrst ﬁeld in the documents. If the server receives a document that does not have the

_id ﬁeld ﬁrst, then the server will move the ﬁeld to the beginning.

• The _id ﬁeld may contain values of any BSON data type (page 12), other than an array.

Warning: To ensure functioning replication, do not store values that are of the BSON regular expression

type in the _id ﬁeld.

The following are common options for storing values for _id:

• Use an ObjectId (page 14).

• Use a natural unique identiﬁer, if available. This saves space and avoids an additional index.

• Generate an auto-incrementing number. See Create an Auto-Incrementing Sequence Field (page 153).

• Generate a UUID in your application code. For a more efﬁcient storage of the UUID values in the collection

and in the _id index, store the UUID as a value of the BSON BinData type.

Index keys that are of the BinData type are more efﬁciently stored in the index if:

– the binary subtype value is in the range of 0-7 or 128-135, and

– the length of the byte array is: 0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16, 20, 24, or 32.

• Use your driver’s BSON UUID facility to generate UUIDs. Be aware that driver implementations may imple-

ment UUID serialization and deserialization logic differently, which may not be fully compatible with other

drivers. See your driver documentation

for information concerning UUID interoperability.

Note: Most MongoDB driver clients will include the _id ﬁeld and generate an ObjectId before sending the insert

operation to MongoDB; however, if the client sends a document without an _id ﬁeld, the mongod will add the _id

ﬁeld and generate the ObjectId.

Other Uses of the Document Structure

In addition to deﬁning data records, MongoDB uses the document structure throughout, including but not limited to:

• Query ﬁlters or speciﬁcations (page 120). Query ﬁlter documents specify the conditions that determine which

records to select for read, update, and delete operations.

You can use <field>:<value> expressions to specify the equality condition and query operator ex-

pressions.

{

<field1>: <value1>,

<field2>: { <operator>: <value> },

...

}

For examples, see Query ﬁlters or speciﬁcations (page 120).

https://api.mongodb.org/

1.2. Key Features 11

MongoDB Documentation, Release 3.2.4

The comparison treats a non-existent ﬁeld as it would an empty BSON Object. As such, a sort on the a ﬁeld in

documents { } and { a: null } would treat the documents as equivalent in sort order.

With arrays, a less-than comparison or an ascending sort compares the smallest element of arrays, and a greater-than

comparison or a descending sort compares the largest element of the arrays. As such, when comparing a ﬁeld whose

value is a single-element array (e.g. [ 1 ]) with non-array ﬁelds (e.g. 2), the comparison is between 1 and 2. A

comparison of an empty array (e.g. [ ]) treats the empty array as less than null or a missing ﬁeld.

MongoDB sorts BinData in the following order:

1. First, the length or size of the data.

2. Then, by the BSON one-byte subtype.

3. Finally, by the data, performing a byte-by-byte comparison.

The following sections describe special considerations for particular BSON types.

ObjectId

ObjectIds are small, likely unique, fast to generate, and ordered. ObjectId values consists of 12-bytes, where the ﬁrst

four bytes are a timestamp that reﬂect the ObjectId’s creation, speciﬁcally:

• a 4-byte value representing the seconds since the Unix epoch,

• a 3-byte machine identiﬁer,

• a 2-byte process id, and

• a 3-byte counter, starting with a random value.

In MongoDB, documents stored in a collection require a unique _id ﬁeld that acts as a primary key. If the _id ﬁeld is

unspeciﬁed in documents, MongoDB uses ObjectIds as the default value for the _id ﬁeld; i.e. if a document does not

contain a top-level _id ﬁeld, the MongoDB driver adds the _id ﬁeld that holds an ObjectId.

In addition, if the mongod receives a document to insert that does not contain an _id ﬁeld, mongod will add the

_id ﬁeld that holds an ObjectId.

MongoDB clients should add an _id ﬁeld with a unique ObjectId. Using ObjectIds for the _id ﬁeld provides the

following additional beneﬁts:

• in the mongo shell, you can access the creation time of the ObjectId, using the

ObjectId.getTimestamp() method.

• sorting on an _id ﬁeld that stores ObjectId values is roughly equivalent to sorting by creation time.

Important: The relationship between the order of ObjectId values and generation time is not strict within a

single second. If multiple systems, or multiple processes or threads on a single system generate values, within a

single second; ObjectId values do not represent a strict insertion order. Clock skew between clients can also

result in non-strict ordering even for values because client drivers generate ObjectId values.

See also:

ObjectId()

String

BSON strings are UTF-8. In general, drivers for each programming language convert from the language’s string format

to UTF-8 when serializing and deserializing BSON. This makes it possible to store most international characters in

14 Chapter 1. Introduction to MongoDB

MongoDB Documentation, Release 3.2.4

BSON strings with ease.

In addition, MongoDB $regex queries support UTF-8 in the regex string.

Timestamps

BSON has a special timestamp type for internal MongoDB use and is not associated with the regular Date (page 15)

type. Timestamp values are a 64 bit value where:

• the ﬁrst 32 bits are a time_t value (seconds since the Unix epoch)

• the second 32 bits are an incrementing ordinal for operations within a given second.

Within a single mongod instance, timestamp values are always unique.

In replication, the oplog has a ts ﬁeld. The values in this ﬁeld reﬂect the operation time, which uses a BSON

timestamp value.

Note: The BSON timestamp type is for internal MongoDB use. For most cases, in application development, you will

want to use the BSON date type. See Date (page 15) for more information.

If you insert a document containing an empty BSON timestamp in a top-level ﬁeld, the MongoDB server will replace

that empty timestamp with the current timestamp value. For example, if you create an insert a document with a

timestamp value, as in the following operation:

var a = new Timestamp();

db.test.insert( { ts: a } );

Then, the db.test.find() operation will return a document that resembles the following:

{ "_id" : ObjectId("542c2b97bac0595474108b48"), "ts" : Timestamp(1412180887, 1) }

If ts were a ﬁeld in an embedded document, the server would have left it as an empty timestamp value.

Changed in version 2.6: Previously, the server would only replace empty timestamp values in the ﬁrst two ﬁelds,

including _id, of an inserted document. Now MongoDB will replace any top-level ﬁeld.

Date

BSON Date is a 64-bit integer that represents the number of milliseconds since the Unix epoch (Jan 1, 1970). This

results in a representable date range of about 290 million years into the past and future.

The ofﬁcial BSON speciﬁcation

refers to the BSON Date type as the UTC datetime.

Changed in version 2.0: BSON Date type is signed.

Negative values represent dates before 1970.

Example

Construct a Date using the new Date() constructor in the mongo shell:

var mydate1 = new Date()

Given strings using UTF-8 character sets, using sort() on strings will be reasonably correct. However, because internally sort() uses the

C++ strcmp api, the sort order may handle some characters incorrectly.

http://bsonspec.org/#/speciﬁcation

Prior to version 2.0, Date values were incorrectly interpreted as unsigned integers, which affected sorts, range queries, and indexes on Date

ﬁelds. Because indexes are not recreated when upgrading, please re-index if you created an index on Date values with an earlier version, and dates

before 1970 are relevant to your application.

1.2. Key Features 15

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