Database Reference
In-Depth Information
$inc
is as efficient as it is convenient. Because it rarely changes the size of a document,
an
$inc
usually occurs in-place on disk, thus affecting only the value pair specified.
8
As demonstrated in the code for adding products to a shopping cart,
$inc
works
with upserts. For example, you can change the preceding update to an upsert like so:
db.readings.update({_id: 324}, {$inc: {temp: 2.7435}}, true)
If no reading with an
_id
of 324 exists, a new document will be created with said
_id
and a
temp
with the value of the
$inc
, 2.7435.
$set and $unset
If you need to set the value of a particular key in a document, you'll want to use
$set
.
You can set a key to a value having any valid
BSON
type. This means that all of the fol-
lowing updates are possible:
db.readings.update({_id: 324}, {$set: {temp: 97.6}})
db.readings.update({_id: 325}, {$set: {temp: {f: 212, c: 100} })
db.readings.update({_id: 326}, {$set: {temps: [97.6, 98.4, 99.1]}})
If the key being set already exists, then its value will be overwritten; otherwise, a new
key will be created.
$unset
removes the provided key from a document. Here's how to remove the
temp
key from the reading document:
db.readings.update({_id: 324}, {$unset: {temp: 1})
You can also use
$unset
on embedded documents and on arrays. In both cases, you
specify the inner object using dot notation. If you have these two documents in your
collection
{_id: 325, 'temp': {f: 212, c: 100}}
{_id: 326, temps: [97.6, 98.4, 99.1]}
then you can remove the Fahrenheit reading in the first document and the zeroth ele-
ment in the second document like so:
db.readings.update({_id: 325},
{$unset: {'temp.f': 1}})
db.readings.update({_id: 236},
{$pop: {temps: -1})
This dot notation for accessing sub-documents and array elements can also be used
with
$set
.
$rename
If you need to change the name of a key, use
$rename
:
db.readings.update({_id: 324}, {$rename: {'temp': 'temperature'})
You can also rename a sub-document:
db.readings.update({_id: 325}, {$rename: {'temp.f': 'temp.farenheit'})
8
Exceptions to this rule arise when the numeric type changes. If the
$inc
results in a 32-bit integer being con-
verted to a 64-bit integer, then the entire BSON document will have to be rewritten in-place.
