Mongodb tutorial Chapter 07 crud search document

This article will introduce how to use the findOne() and find() Method to find the document .

Use findOne() Method to find a single document

findOne() Method is used to return a single document in the collection that meets the condition , The syntax of this method is as follows ：

db.collection.findOne(query, projection)

findOne() Method contains two optional parameters ：

• query Used to specify a selection criterion ;
• projection Used to specify the returned field .

If omitted query Parameters ,findOne() Return to the first document stored on disk . If omitted projection Parameters , By default, all fields in the document are returned .

If you want to specify whether to return a field , You can specify projection Parameters ：

{
    field1: value, field1: value, ... }

among ,value Set to true perhaps 1 Indicates that the field is returned ; If value Set to false perhaps 0,MongoDB This field will not be returned .

By default ,MongoDB Always returns _id Field . If you do not need to return this field , Can be in projection The parameter specifies _id: 0.

If there are multiple documents that meet the query conditions ,findOne() Only the first document found on the disk will be returned .

Next, let's look at some examples , First create a collection products：

db.products.insertMany([
    {
     "_id" : 1, "name" : "xPhone", "price" : 799, "releaseDate": ISODate("2011-05-14"), "spec" : {
     "ram" : 4, "screen" : 6.5, "cpu" : 2.66 },"color":["white","black"],"storage":[64,128,256]},
    {
     "_id" : 2, "name" : "xTablet", "price" : 899, "releaseDate": ISODate("2011-09-01") , "spec" : {
     "ram" : 16, "screen" : 9.5, "cpu" : 3.66 },"color":["white","black","purple"],"storage":[128,256,512]},
    {
     "_id" : 3, "name" : "SmartTablet", "price" : 899, "releaseDate": ISODate("2015-01-14"), "spec" : {
     "ram" : 12, "screen" : 9.7, "cpu" : 3.66 },"color":["blue"],"storage":[16,64,128]},
    {
     "_id" : 4, "name" : "SmartPad", "price" : 699, "releaseDate": ISODate("2020-05-14"),"spec" : {
     "ram" : 8, "screen" : 9.7, "cpu" : 1.66 },"color":["white","orange","gold","gray"],"storage":[128,256,1024]},
    {
     "_id" : 5, "name" : "SmartPhone", "price" : 599,"releaseDate": ISODate("2022-09-14"), "spec" : {
     "ram" : 4, "screen" : 5.7, "cpu" : 1.66 },"color":["white","orange","gold","gray"],"storage":[128,256]}
 ])

The following example uses findOne() Method to find a collection products The first document in ：

db.products.findOne()

The query returns all fields in the document ：

{
    
  _id: 1,
  name: 'xPhone',
  price: 799,
  releaseDate: ISODate("2011-05-14T00:00:00.000Z"),
  spec: {
     ram: 4, screen: 6.5, cpu: 2.66 },
  color: [ 'white', 'black' ],
  storage: [ 64, 128, 256 ]
}

Omit query Parameter is the same as specifying an empty document parameter ：

db.products.findOne({
    })

The following sentence uses findOne() Method to find the _id be equal to 2 Documents ：

 db.products.findOne({
    _id:2})

The results are as follows ：

{
    
  _id: 2,
  name: 'xTablet',
  price: 899,
  releaseDate: ISODate("2011-09-01T00:00:00.000Z"),
  spec: {
     ram: 16, screen: 9.5, cpu: 3.66 },
  color: [ 'white', 'black', 'purple' ],
  storage: [ 128, 256, 512 ]
}

The following example uses findOne() Method to find the _id be equal to 5 Documents , And only returned _id and name Field ：

db.products.findOne({
     _id: 5}, {
     name: 1})

{
     "_id" : 5, "name" : "SmartPhone" }

As you can see from the returned results ,MongoDB By default _id Field .

If you want to remove from the returned result _id Field , You can specify _id:0, for example ：

db.products.findOne({
      _id: 5 }, {
     name: 1, _id: 0})

{
     "name" : "SmartPhone" }

Use find() Method to find the document

find() Method is used to find documents that meet specified conditions , And return a cursor pointing to these documents （ The pointer ）. The syntax of this method is as follows ：

db.collection.find(query, projection)

find() Method contains two optional parameters ：

• query Used to specify a selection criterion . If you omit this parameter or specify an empty document parameter , All documents in the collection will be returned .
• projection Used to specify the returned field . If this parameter is omitted , All fields in the document will be returned .

By default ,MongoDB Always returns _id Field . If you do not need to return this field , Can be in projection The parameter specifies _id: 0.

because mongo shell Automatic traversal find() Method returns a cursor , We do not need to perform additional operations to get the document in the cursor . By default ,mongo shell Show only before 20 document , Input it Command can display more documents .

Now let's use a new set books As a demonstration ：

db.books.insertMany([
	{
     "_id" : 1, "title" : "Unlocking Android", "isbn" : "1933988673", "categories" : [ "Open Source", "Mobile" ] },
	{
     "_id" : 2, "title" : "Android in Action, Second Edition", "isbn" : "1935182722", "categories" : [ "Java" ] },
	{
     "_id" : 3, "title" : "Specification by Example", "isbn" : "1617290084", "categories" : [ "Software Engineering" ] },
	{
     "_id" : 4, "title" : "Flex 3 in Action", "isbn" : "1933988746", "categories" : [ "Internet" ] },
	{
     "_id" : 5, "title" : "Flex 4 in Action", "isbn" : "1935182420", "categories" : [ "Internet" ] },
	{
     "_id" : 6, "title" : "Collective Intelligence in Action", "isbn" : "1933988312", "categories" : [ "Internet" ] },
	{
     "_id" : 7, "title" : "Zend Framework in Action", "isbn" : "1933988320", "categories" : [ "Web Development" ] },
	{
     "_id" : 8, "title" : "Flex on Java", "isbn" : "1933988797", "categories" : [ "Internet" ] },
	{
     "_id" : 9, "title" : "Griffon in Action", "isbn" : "1935182234", "categories" : [ "Java" ] },
	{
     "_id" : 10, "title" : "OSGi in Depth", "isbn" : "193518217X", "categories" : [ "Java" ] },
	{
     "_id" : 11, "title" : "Flexible Rails", "isbn" : "1933988509", "categories" : [ "Web Development" ] },
	{
     "_id" : 13, "title" : "Hello! Flex 4", "isbn" : "1933988762", "categories" : [ "Internet" ] },
	{
     "_id" : 14, "title" : "Coffeehouse", "isbn" : "1884777384", "categories" : [ "Miscellaneous" ] },
	{
     "_id" : 15, "title" : "Team Foundation Server 2008 in Action", "isbn" : "1933988592", "categories" : [ "Microsoft .NET" ] },
	{
     "_id" : 16, "title" : "Brownfield Application Development in .NET", "isbn" : "1933988711", "categories" : [ "Microsoft" ] },
	{
     "_id" : 17, "title" : "MongoDB in Action", "isbn" : "1935182870", "categories" : [ "Next Generation Databases" ] },
	{
     "_id" : 18, "title" : "Distributed Application Development with PowerBuilder 6.0", "isbn" : "1884777686", "categories" : [ "PowerBuilder" ] },
	{
     "_id" : 19, "title" : "Jaguar Development with PowerBuilder 7", "isbn" : "1884777864", "categories" : [ "PowerBuilder", "Client-Server" ] },
	{
     "_id" : 20, "title" : "Taming Jaguar", "isbn" : "1884777686", "categories" : [ "PowerBuilder" ] },
	{
     "_id" : 21, "title" : "3D User Interfaces with Java 3D", "isbn" : "1884777902", "categories" : [ "Java", "Computer Graphics" ] },
	{
     "_id" : 22, "title" : "Hibernate in Action", "isbn" : "193239415X", "categories" : [ "Java" ] },
	{
     "_id" : 23, "title" : "Hibernate in Action (Chinese Edition)", "categories" : [ "Java" ] },
	{
     "_id" : 24, "title" : "Java Persistence with Hibernate", "isbn" : "1932394885", "categories" : [ "Java" ] },
	{
     "_id" : 25, "title" : "JSTL in Action", "isbn" : "1930110529", "categories" : [ "Internet" ] },
	{
     "_id" : 26, "title" : "iBATIS in Action", "isbn" : "1932394826", "categories" : [ "Web Development" ] },
	{
     "_id" : 27, "title" : "Designing Hard Software", "isbn" : "133046192", "categories" : [ "Object-Oriented Programming", "S" ] },
	{
     "_id" : 28, "title" : "Hibernate Search in Action", "isbn" : "1933988649", "categories" : [ "Java" ] },
	{
     "_id" : 29, "title" : "jQuery in Action", "isbn" : "1933988355", "categories" : [ "Web Development" ] },
	{
     "_id" : 30, "title" : "jQuery in Action, Second Edition", "isbn" : "1935182323", "categories" : [ "Java" ] }
]);

The following example uses find() Method returns a collection books All documents in ：

db.books.find()

mongo shell It shows the front 20 document , Contains all fields . Input it Command and return , You will see the following 20 document .

The following example returns _id be equal to 10 Documents , Contains all fields of the document ：

db.books.find({
    _id: 10})

[
  {
    
    _id: 10,
    title: 'OSGi in Depth',
    isbn: '193518217X',
    categories: [ 'Java' ]
  }
]

The following example returns category be equal to “Java” All documents of , The returned result contains _id、title as well as isbn Three fields ：

db.books.find({
     categories: 'Java'}, {
     title: 1,isbn: 1})

[
  {
    
    _id: 2,
    title: 'Android in Action, Second Edition',
    isbn: '1935182722'
  },
  {
     _id: 9, title: 'Griffon in Action', isbn: '1935182234' },
  {
     _id: 10, title: 'OSGi in Depth', isbn: '193518217X' },
  {
    
    _id: 21,
    title: '3D User Interfaces with Java 3D',
    isbn: '1884777902'
  },
  {
     _id: 22, title: 'Hibernate in Action', isbn: '193239415X' },
  {
     _id: 23, title: 'Hibernate in Action (Chinese Edition)' },
  {
    
    _id: 24,
    title: 'Java Persistence with Hibernate',
    isbn: '1932394885'
  },
  {
     _id: 28, title: 'Hibernate Search in Action', isbn: '1933988649' },
  {
    
    _id: 30,
    title: 'jQuery in Action, Second Edition',
    isbn: '1935182323'
  }
]

Use the projection operation to return the specified field

stay MongoDB in , Projection （projection） Indicates that the specified field is returned in the query .

By default find() and findOne() Method will return all fields in the document , But in most cases, we don't need to query all the fields .

If you want to choose to return some fields , You can specify these fields in a document and pass the document as a parameter to find() and findOne() Method . This parameter document is called projection document . The syntax of specifying the return field is as follows ：

{
     <field>: value, ...}

If value Set to 1 perhaps true, Indicates the return field ; If value Set to 0 perhaps false, Indicates that this field is not returned . If the projection document is empty （{}）, Indicates that all fields are returned .

For fields in embedded documents , You can use point numbers to specify ：

{
     "<embeddedDocument>.<field>": value, ... }

The following example returns _id be equal to 1 In the document name、price as well as _id Field , It also returns the embedded document spec Medium screen Field ：

db.products.find({
    _id:1}, {
    
    name: 1,
    price: 1,
    "spec.screen": 1
})

The output is as follows ：

[ {
     _id: 1, name: 'xPhone', price: 799, spec: {
     screen: 6.5 } } ]

MongoDB 4.4 And later versions also support specifying the returned fields in a nested form ：

db.products.find({
    _id:1}, {
    
    name: 1,
    price: 1,
    spec : {
     screen: 1 }
})

A similar , For fields in the array , You can also specify ：

{
     "<arrayField>.field": value, ...}

The following example returns _id、name And arrays inventory Medium qty Field ：

db.products.find({
    }, {
    
    name: 1,
    "inventory.qty": 1
});

The output is as follows ：

[
  {
     _id: 1, name: 'xPhone', inventory: [ {
     qty: 1200 } ] },
  {
     _id: 2, name: 'xTablet', inventory: [ {
     qty: 300 } ] },
  {
    
    _id: 3, name: 'SmartTablet', inventory: [ {
     qty: 400 }, {
     qty: 200 } ]
  },
  {
     _id: 4, name: 'SmartPad', inventory: [ {
     qty: 1200 } ] },
  {
     _id: 5, name: 'SmartPhone' }
]