4.Data Types in the mongo Shell-官方文档摘录

总结:

1.MongoDB 的BSON格式支持额外的数据类型

2 Date 对象内部存储64位字节存整数,存储使用NumberLong()这个类来存，使用NumberInt()存32位整数,128位十进制的用NumberDecimal()存储(这个函数是为了方便的存储财政数据)

db.collection.insertOne( { _id: 10, calc: NumberLong("2090845886852") } )

3 时间的三个方法

test@gzxkvm52> var myDateString=Date()
test@gzxkvm52> myDateString
Mon Jul 24 2017 11:16:12 GMT+0800 (CST)
test@gzxkvm52> var myDate = new Date();
test@gzxkvm52> var myDateInitUsingISODateWrapper = ISODate();
test@gzxkvm52> myDate
ISODate("2017-07-24T03:17:54.900Z")
test@gzxkvm52> myDateInitUsingISODateWrapper
ISODate("2017-07-24T03:18:02.774Z")

4 使用NunberDecimal需要注意的问题(需要使用双引号，否则会有精度问题)

test@gzxkvm52> aa = NumberDecimal("12.444444")
NumberDecimal("12.444444")
test@gzxkvm52> aa
NumberDecimal("12.444444")
test@gzxkvm52> bb = NumberDecimal(12.444444)
NumberDecimal("12.4444440000000")
test@gzxkvm52> bb
NumberDecimal("12.4444440000000")
test@gzxkvm52> bb = NumberDecimal(12.464444)
NumberDecimal("12.4644440000000")
test@gzxkvm52> bb
NumberDecimal("12.4644440000000")
test@gzxkvm52> cc = NumberDecimal(9999999.4999999999
... 
... )
NumberDecimal("9999999.50000000")
test@gzxkvm52> cc
NumberDecimal("9999999.50000000")
test@gzxkvm52> dd = NumberDecimal("9999999.4999999999")
NumberDecimal("9999999.4999999999")
test@gzxkvm52> dd
NumberDecimal("9999999.4999999999")

5 使用instanceof判断数据类型(真假)

MongoDB BSON provides support for additional data types than JSON. Drivers provide native support for these data types in host languages and the mongo shell also provides several helper classes to support the use of these data types in the mongo JavaScript shell. See the Extended JSON reference for additional information.

Types

Date

The mongo shell provides various methods to return the date, either as a string or as a Date object:

• Date() method which returns the current date as a string.
• new Date() constructor which returns a Date object using the ISODate() wrapper.
• ISODate() constructor which returns a Date object using the ISODate() wrapper.

Internally, Date objects are stored as a 64 bit integer representing the number of milliseconds since the Unix epoch (Jan 1, 1970), which results in a representable date range of about 290 millions years into the past and future.

Return Date as a String

To return the date as a string, use the Date() method, as in the following example:

var myDateString = Date();

To print the value of the variable, type the variable name in the shell, as in the following:

myDateString

The result is the value of myDateString:

Wed Dec 19 2012 01:03:25 GMT-0500 (EST)

To verify the type, use the typeof operator, as in the following:

typeof myDateString

The operation returns string.

Return Date

The mongo shell wraps objects of Date type with the ISODate helper; however, the objects remain of typeDate.

The following example uses both the new Date() constructor and the ISODate() constructor to returnDate objects.

var myDate = new Date();
var myDateInitUsingISODateWrapper = ISODate();

You can use the new operator with the ISODate() constructor as well.

To print the value of the variable, type the variable name in the shell, as in the following:

myDate

The result is the Date value of myDate wrapped in the ISODate() helper:

ISODate("2012-12-19T06:01:17.171Z")

To verify the type, use the instanceof operator, as in the following:

myDate instanceof Date
myDateInitUsingISODateWrapper instanceof Date

The operation returns true for both.

ObjectId

The mongo shell provides the ObjectId() wrapper class around the ObjectId data type. To generate a new ObjectId, use the following operation in the mongo shell:

new ObjectId

SEE

ObjectId

NumberLong

The mongo shell treats all numbers as floating-point values by default. The mongo shell provides theNumberLong() wrapper to handle 64-bit integers.

The NumberLong() wrapper accepts the long as a string:

NumberLong("2090845886852")

The following examples use the NumberLong() wrapper to write to the collection:

db.collection.insertOne( { _id: 10, calc: NumberLong("2090845886852") } )
db.collection.updateOne( { _id: 10 },
                      { $set:  { calc: NumberLong("2555555000000") } } )
db.collection.updateOne( { _id: 10 },
                      { $inc: { calc: NumberLong(5) } } )

Retrieve the document to verify:

db.collection.findOne( { _id: 10 } )

In the returned document, the calc field contains a NumberLong object:

{ "_id" : 10, "calc" : NumberLong("2555555000005") }

If you use the $inc to increment the value of a field that contains a NumberLong object by a float, the data type changes to a floating point value, as in the following example:

1. Use $inc to increment the calc field by 5, which the mongo shell treats as a float:

   db.collection.updateOne( { _id: 10 },
                         { $inc: { calc: 5 } } )
   

2. Retrieve the updated document:

   db.collection.findOne( { _id: 10 } )
   

   In the updated document, the calc field contains a floating point value:

   { "_id" : 10, "calc" : 2555555000010 }
   

NumberInt

The mongo shell treats all numbers as floating-point values by default. The mongo shell provides theNumberInt() constructor to explicitly specify 32-bit integers.

NumberDecimal

New in version 3.4.

The mongo shell treats all numbers as 64-bit floating-point double values by default. The mongo shell provides the NumberDecimal() constructor to explicitly specify 128-bit decimal-based floating-point values capable of emulating decimal rounding with exact precision. This functionality is intended for applications that handle monetary data, such as financial, tax, and scientific computations.

The decimal BSON type uses the IEEE 754 decimal128 floating-point numbering format which supports 34 decimal digits (i.e. significant digits) and an exponent range of −6143 to +6144.

The NumberDecimal() constructor accepts the decimal value as a string:

NumberDecimal("1000.55")

The value is stored in the database as follows:

NumberDecimal("1000.55")

The NumberDecimal() constructor also accepts double values from the mongo shell (i.e. without quotes), although this is not recommended due to the risk of losing precision. The constructor creates a binary-baseddouble precision representation of the decimal-based parameter (potentially losing precision), then converts that value to a decimal value with a precision of 15 digits. The following example passes the value implicitly as a double and shows how it is created with a precision of 15 digits:

NumberDecimal(1000.55)

The value is stored in the database as follows:

NumberDecimal("1000.55000000000")

The following example passes the value implicitly as a double and shows how a loss of precision can occur:

NumberDecimal(9999999.4999999999)

The value is stored in the database as follows:

NumberDecimal("9999999.50000000")

NOTE

To use the decimal data type with a MongoDB driver, be sure to use a driver version that supports it.

Equality and Sort Order

Values of the decimal type are compared and sorted with other numeric types based on their actual numeric value. Numeric values of the binary-based double type generally have approximate representations of decimal-based values and may not be exactly equal to their decimal representations, so use theNumberDecimal() constructor when checking the equality of decimal values. Consider the following examples with the following documents in the numbers collection:

{ "_id" : 1, "val" : NumberDecimal( "9.99" ), "description" : "Decimal" }
{ "_id" : 2, "val" : 9.99, "description" : "Double" }
{ "_id" : 3, "val" : 10, "description" : "Double" }
{ "_id" : 4, "val" : NumberLong(10), "description" : "Long" }
{ "_id" : 5, "val" : NumberDecimal( "10.0" ), "description" : "Decimal" }

When the queries from the table below are plugged into the db.numbers.find(<query>) method, the following results are returned:

Query	Results
{ “val”: 9.99 }	{ “_id”: 2, “val”: 9.99, “description”: “Double” }
{ “val”: NumberDecimal( “9.99” ) }	{ “_id”: 1, “val”: NumberDecimal( “9.99” ), “description”: “Decimal” }
{ val: 10 }	{ “_id”: 3, “val”: 10, “description”: “Double” } { “_id”: 4, “val”: NumberLong(10), “description”: “Long” } { “_id”: 5, “val”: NumberDecimal( “10.0” ), “description”: “Decimal” }
{ val: NumberDecimal( “10” ) }	{ “_id”: 3, “val”: 10, “description”: “Double” } { “_id”: 4, “val”: NumberLong(10), “description”: “Long” } { “_id”: 5, “val”: NumberDecimal( “10.0” ), “description”: “Decimal” }

The first query, { "val": 9.99 }, implicitly searches for the double representation of 9.99 which is not equal to the decimal representation of the value.

The NumberDecimal() constructor is used to query for the document with the decimal representation of9.99. Values of the double type are excluded because they do not match the exact value of the decimalrepresentation of 9.99.

Matching values of all numeric types are returned when querying for whole numbers. For example, querying for a double representation of 10 will include a decimal representation of 10.0 in the results and vice versa.

Checking for decimal Type

To test for decimal type, use the $type operator with the string alias "decimal" or 19, the numeric code for the decimal type.

db.inventory.find( { price: { $type: "decimal" } } )

Check Types in the mongo Shell

To determine the type of fields, the mongo shell provides the instanceof and typeof operators.

instanceof

instanceof returns a boolean to test if a value is an instance of some type.

For example, the following operation tests whether the _id field is an instance of type ObjectId:

mydoc._id instanceof ObjectId

The operation returns true.

typeof

typeof returns the type of a field.

For example, the following operation returns the type of the _id field:

typeof mydoc._id

In this case typeof will return the more generic object type rather than ObjectId type.