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===========
Collections
===========
MongoDB stores documents in collections. If a collection does not
exist, MongoDB creates the collection when you first store data for
that collection.
You can also explicitly create a collection with various options,
such as setting the maximum size or the documentation validation rules.
Capped Collection
-----------------
Capped collections have maximum size or document counts that prevent
them from growing beyond maximum thresholds. All capped collections must
specify a maximum size and may also specify a maximum document count.
MongoDB removes older documents if a collection reaches the maximum size
limit before it reaches the maximum document count.
To create a :manual:`capped collection </core/capped-collections/>` ,
use the ``createCollection`` method and specify ``'capped' : true``.
.. code-block:: js
const { MongoClient } = require('mongodb');
// Connection URL
const url = 'mongodb://localhost:27017';
// Create a new MongoClient
const client = new MongoClient(url);
async function main(client) {
const db = client.db('test');
await db.createCollection('myCollection', {
capped: true,
size: 100000,
max: 5000
});
console.log('Collection created.');
}
// Function to connect to the server and run your code
async function run() {
try {
// Connect the client to the server
await client.connect();
console.log('Connected successfully to server');
await main(client);
} finally {
// Ensures that the client will close when you finish/error
await client.close();
}
}
// Runs your code
run();
Document Validation
-------------------
Collections with :manual:`validation </core/schema-validation/>`
compare each inserted or updated
document against the criteria specified in the validator option.
Depending on the ``validationLevel`` and ``validationAction``\ , MongoDB
either returns a warning, or refuses to insert or update the document
if it fails to meet the specified criteria.
The following example creates a ``contacts`` collection with a validator
that specifies that inserted or updated documents should match at
least one of three following conditions:
* the ``phone`` field is a string
* the ``email`` field matches the regular expression
* the ``status`` field is either ``Unknown`` or ``Incomplete``.
.. code-block:: js
async function main(client) {
const db = client.db('test');
await db.createCollection('myCollection', {
validator: {
$or: [
{ phone: { $type: 'string' } },
{ email: { $regex: /@mongodb\.com$/ } },
{ status: { $in: ['Unknown', 'Incomplete'] } }
]
}
});
console.log('Collection created.');
}
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