Updating Arrays

1. Adding to Arrays

Operator	Behavior
$push	Append element
$push + $each	Append multiple
$addToSet	Append if not already present

db.users.updateOne({_id: id}, { $push: { tags: "vip" } });

Form	Effect
`$push: { tags: { $each: ["a","b"] } }`	Append multiple
`$addToSet: { tags: { $each: ["a","b"] } }`	Unique multi-add
With $sort / $slice	Maintain bounded sorted array

Operator	Effect
$pull	Remove elements matching condition
$pullAll	Remove elements equal to any in list
$pop	Remove first (-1) or last (1)

db.users.updateMany({}, { $pull: { sessions: { expiresAt: { $lt: new Date() } } } });

Form	Effect
`$pop: {tags: 1}`	Remove last element
`$pop: {tags: -1}`	Remove first element

Aspect	Detail
$addToSet	Equality compare entire value (incl. nested docs by exact match)
Subdoc gotcha	{a:1,b:2} ≠ {b:2,a:1} for some BSON forms; use canonical field order

Form	Effect
`{"items.$": value}`	First matched element from query filter
Requires	Array field in filter

db.orders.updateOne(
  { _id: id, "items.sku": "A" },
  { $set: { "items.$.qty": 5 } }
);

Form	Effect
`{"scores.$[]": newValue}`	Update every element
`{"scores.$[]": {$mul: 1.1}}`	Use with operators

Form	Effect
`$[elem]`	Update elements matching arrayFilters entry
arrayFilters	Array of filter docs per identifier
Multiple	Multiple identifiers per update allowed

db.classes.updateMany(
  {},
  { $set: { "students.$[s].passed": true } },
  { arrayFilters: [ { "s.grade": { $gte: 70 } } ] }
);

Form	Effect
`$push: { items: { $each: [...], $sort: {ts: -1} } }`	Sort subdocs by field
`$sort: 1`	Sort scalar array ascending

Pattern	Form
Bounded queue	`$push: {recent: {$each: [x], $slice: -50}}`
Top-N by score	$each + $sort + $slice

Form	Effect
`$pullAll: { tags: ["x","y"] }`	Remove every element equal to any listed value
vs $pull	$pullAll uses equality; $pull uses match expression