Getting Started起步

Switch Database

Within the shell, db refers to your current database. Type db to display the current database.

db

The operation should return test, which is the default database.

To switch databases, type use <db>. For example, to switch to the examples database:

use examples

You do not need to create the database before you switch. MongoDB creates the database when you first store data in that database (such as create the first collection in the database).

To verify that your database is now examples, type db in the shell above.

db

To create a collection in the database, see the next tab.

Populate a Collection (Insert)

MongoDB stores documents in collections. Collections are analogous to tables in relational databases. If a collection does not exist, MongoDB creates the collection when you first store data for that collection.

The following example uses the db.collection.insertMany() method to insert new documents into the movies collection. You can copy and paste the example into the shell above.

db.movies.insertMany([
   {
      title: 'Titanic',
      year: 1997,
      genres: [ 'Drama', 'Romance' ],
      rated: 'PG-13',
      languages: [ 'English', 'French', 'German', 'Swedish', 'Italian', 'Russian' ],
      released: ISODate("1997-12-19T00:00:00.000Z"),
      awards: {
         wins: 127,
         nominations: 63,
         text: 'Won 11 Oscars. Another 116 wins & 63 nominations.'
      },
      cast: [ 'Leonardo DiCaprio', 'Kate Winslet', 'Billy Zane', 'Kathy Bates' ],
      directors: [ 'James Cameron' ]
   },
   {
      title: 'The Dark Knight',
      year: 2008,
      genres: [ 'Action', 'Crime', 'Drama' ],
      rated: 'PG-13',
      languages: [ 'English', 'Mandarin' ],
      released: ISODate("2008-07-18T00:00:00.000Z"),
      awards: {
         wins: 144,
         nominations: 106,
         text: 'Won 2 Oscars. Another 142 wins & 106 nominations.'
      },
      cast: [ 'Christian Bale', 'Heath Ledger', 'Aaron Eckhart', 'Michael Caine' ],
      directors: [ 'Christopher Nolan' ]
   },
   {
      title: 'Spirited Away',
      year: 2001,
      genres: [ 'Animation', 'Adventure', 'Family' ],
      rated: 'PG',
      languages: [ 'Japanese' ],
      released: ISODate("2003-03-28T00:00:00.000Z"),
      awards: {
         wins: 52,
         nominations: 22,
         text: 'Won 1 Oscar. Another 51 wins & 22 nominations.'
      },
      cast: [ 'Rumi Hiiragi', 'Miyu Irino', 'Mari Natsuki', 'Takashi Naitè' ],
      directors: [ 'Hayao Miyazaki' ]
   },
   {
      title: 'Casablanca',
      genres: [ 'Drama', 'Romance', 'War' ],
      rated: 'PG',
      cast: [ 'Humphrey Bogart', 'Ingrid Bergman', 'Paul Henreid', 'Claude Rains' ],
      languages: [ 'English', 'French', 'German', 'Italian' ],
      released: ISODate("1943-01-23T00:00:00.000Z"),
      directors: [ 'Michael Curtiz' ],
      awards: {
         wins: 9,
         nominations: 6,
         text: 'Won 3 Oscars. Another 6 wins & 6 nominations.'
      },
      lastupdated: '2015-09-04 00:22:54.600000000',
      year: 1942
   }
])

The operation returns a document that contains the acknowledgement indicator and an array that contains the _id of each successfully inserted documents.

To verify the insert, you can query the collection (See the next tab).

Select All Documents

To select the documents from a collection, you can use the db.collection.find() method. To select all documents in the collection, pass an empty document as the query filter document to the method.

In the shell, copy and paste the following to return all documents in the movies collection.

db.movies.find( { } )

Filter Data with Comparison Operators

For an equality match (<field> equals <value>), specify <field>: <value> in the query filter document and pass to the db.collection.find() method.

• In the shell, run the following query to find movies that were directed by Christopher Nolan:

  db.movies.find( { "directors": "Christopher Nolan" } );

You can use comparison operators to perform more advanced queries:

• Run the following query to return movies that were released before the year 2000:

  db.movies.find( { "released": { $lt: ISODate("2000-01-01") } } );

• Run the following query to return movies that won more than 100 awards:

  db.movies.find( { "awards.wins": { $gt: 100 } } );

• Run the following query to return movies where the languages array contains either Japanese or Mandarin:

  db.movies.find( { "languages": { $in: [ "Japanese", "Mandarin" ] } } )

  Tip

  See also:

  Query and Projection Operators

Specify Fields to Return (Projection)

To specify fields to return, pass a projection document to the db.collection.find(<query document>, <projection document>) method. In the projection document, specify:

• <field>: 1 to include a field in the returned documents

• <field>: 0 to exclude a field in the returned documents

In the shell, run the following query to return the id, title, directors, and year fields from all documents in the movies collection:

db.movies.find( { }, { "title": 1, "directors": 1, "year": 1 } );

You do not have to specify the _id field to return the field. It returns by default. To exclude the field, set it to 0 in the projection document. For example, run the following query to return only the title, and the genres fields in the matching documents:

db.movies.find( { }, { "_id": 0, "title": 1, "genres": 1 } );

Aggregate Data ($group)

You can use aggregation to group values from multiple documents together and return a single result. Aggregation in MongoDB is performed with an aggregation pipeline.

While find() operations are useful for data retrieval, the aggregation pipeline allows you to manipulate data, perform calculations, and write more expressive queries than simple CRUD operations.

In the shell, run the following aggregation pipeline to count the number of occurrences of each genre value:

db.movies.aggregate( [
   { $unwind: "$genres" },
   {
     $group: {
       _id: "$genres",
       genreCount: { $count: { } }
     }
   },
   { $sort: { "genreCount": -1 } }
] )

The pipeline uses:

• $unwind to output a document for each element in the genres array.

• $group and the $count accumulator to count the number of occurrences of each genre. This value is stored in the genreCount field.

• $sort to sort the resulting documents by the genreCount field in descending order.