Updates, atomic operations, and deletes - MongoDB in Action - page 121

Using $unset with arrays
Note that using $unset on individual array elements may not work exactly like you
want it to. Instead of removing the element altogether, it merely sets that element's
value to null . To completely remove an array element, see the $pull and $pop
operators.
db.readings.update({_id: 325}, {$unset: {'temp.f': 1})
db.readings.update({_id: 326}, {$unset: {'temp.0': 1})
A RRAY UPDATE OPERATORS
The centrality of arrays in MongoDB's document model should be apparent. Naturally,
MongoDB provides a handful of update operators that apply exclusively to arrays.
$push and $pushAll
If you need to append values to an array, $push and $pushAll are your friends. The
first of these, $push , will add a single value to an array, whereas $pushAll supports
adding a list of values. For example, adding a new tag to the shovel product is easy
enough:
db.products.update({slug: 'shovel'}, {$push: {'tags': 'tools'}})
If you need to add a few tags in the same update, that's not a problem either:
db.products.update({slug: 'shovel'},
{$pushAll: {'tags': ['tools', 'dirt', 'garden']}})
Note you can push values of any type onto an array, not just scalars. For an example of
this, see the code in the previous section that pushed a product onto the shopping
cart's line items array.
$addToSet and $each
$addToSet also appends a value to an array but does so in a more discerning way: the
value is added only if it doesn't already exist in the array. Thus, if your shovel has
already been tagged as a tool then the following update won't modify the document
at all:
db.products.update({slug: 'shovel'}, {$addToSet: {'tags': 'tools'}})
If you need to add more than one value to an array uniquely in the same operation,
then you must use $addToSet with the $each operator. Here's how that looks:
db.products.update({slug: 'shovel'},
{$addToSet: {'tags': {$each: ['tools', 'dirt', 'steel']}}})
Only those values in the $each that don't already exist in tags will be appended.
$pop
The most elementary way to remove an item from an array is with the $pop operator. If
$push appends an item to an array, a subsequent $pop will remove that last item
pushed. Though it's frequently used with $push , you can use $pop on its own. If your