- Introduction to MongoDB >
- Databases and Collections
Databases and Collections¶
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Overview¶
MongoDB stores data records as documents (specifically BSON documents) which are gathered together in collections. A database stores one or more collections of documents.
Databases¶
In MongoDB, databases hold one or more collections of documents. To
select a database to use, in the mongo shell, issue the
use <db> statement, as in the following example:
Create a Database¶
If a database does not exist, MongoDB creates the database when you
first store data for that database. As such, you can switch to a
non-existent database and perform the following operation in the
mongo shell:
The insertOne() operation creates both the
database myNewDB and the collection myNewCollection1 if they do
not already exist. Be sure that both the database and collection names
follow MongoDB Naming Restrictions.
Collections¶
MongoDB stores documents in collections. Collections are analogous to tables in relational databases.
Create a Collection¶
If a collection does not exist, MongoDB creates the collection when you first store data for that collection.
Both the insertOne() and the
createIndex() operations create their
respective collection if they do not already exist. Be sure that the
collection name follows MongoDB Naming Restrictions.
Explicit Creation¶
MongoDB provides the db.createCollection() method to
explicitly create a collection with various options, such as setting
the maximum size or the documentation validation rules. If you are not
specifying these options, you do not need to explicitly create the
collection since MongoDB creates new collections when you first store
data for the collections.
To modify these collection options, see collMod.
Document Validation¶
New in version 3.2.
By default, a collection does not require its documents to have the same schema; i.e. the documents in a single collection do not need to have the same set of fields and the data type for a field can differ across documents within a collection.
Starting in MongoDB 3.2, however, you can enforce document validation rules for a collection during update and insert operations. See Schema Validation for details.
Modifying Document Structure¶
To change the structure of the documents in a collection, such as add new fields, remove existing fields, or change the field values to a new type, update the documents to the new structure.
Unique Identifiers¶
New in version 3.6.
Note
The featureCompatibilityVersion must be set to "3.6" or greater. For
more information, see View FeatureCompatibilityVersion.
Collections are assigned an immutable UUID. The collection UUID remains the same across all members of a replica set and shards in a sharded cluster.
To retrieve the UUID for a collection, run either the
listCollections command
or the db.getCollectionInfos() method.