MongoDB Array Update Operators

These operators modify values inside an array field, rather than replacing the whole array.

Operator	Description
$	Placeholder for the first array element that matched the query
$[]	Placeholder for every element in the array
$[<identifier>]	Placeholder for elements matching an arrayFilters condition
$addToSet	Adds a value only if it isn't already present
$pop	Removes the first or last element
$pull	Removes all elements matching a condition
$pullAll	Removes all elements matching a list of exact values
$push	Appends a value to the array

The Positional $ Operator

Updates the first array element that matched your query condition, without you needing to know its index.

db.employees.updateOne(
  { empid: 202, deptwork: 2 },
  { $set: { "deptwork.$": 4 } }
)
// replaces the "2" inside deptwork with "4"

If there are multiple matching elements, only the first one is changed.

$[] — Update Every Element

db.employees.updateOne(
  { empid: 202 },
  { $inc: { "deptwork.$[]": 2 } }
)
// adds 2 to every number in the deptwork array

$[] — Update Filtered Elements

Combine with arrayFilters to update only elements that match a condition — and unlike the plain $, this updates all matches, not just the first.

db.employees.updateOne(
  { empid: 202 },
  { $set: { "deptwork.$[ele]": 8 } },
  { arrayFilters: [{ "ele": 6 }] }
)
// every "6" in deptwork becomes "8"

$addToSet — Add Without Duplicates

db.employees.updateOne({ empid: 202 }, { $addToSet: { deptwork: 10 } })
// inserts 10

db.employees.updateOne({ empid: 202 }, { $addToSet: { deptwork: 10 } })
// no-op — 10 is already there

Passing a plain array inserts the whole array as one element — use $each to add multiple individual values:

db.employees.updateOne(
  { empid: 204 },
  { $addToSet: { deptwork: { $each: [1, 2, 3] } } }
)

$pop — Remove First or Last Element

db.employees.updateOne({ empid: 202 }, { $pop: { deptwork: 1 } })   // removes the LAST element
db.employees.updateOne({ empid: 202 }, { $pop: { deptwork: -1 } })  // removes the FIRST element

$pull — Remove Matching Elements

db.employees.updateOne({ empid: 204 }, { $pull: { deptwork: 3 } })
// removes the value 3 wherever it appears

db.employees.updateOne({ empid: 204 }, { $pull: { deptwork: { $in: [1, 2] } } })
// removes 1 AND 2

$pullAll — Remove an Exact List of Values

db.employees.updateOne({ empid: 202 }, { $pullAll: { deptwork: [10, 11] } })

$push — Append to an Array

db.employees.updateOne({ empid: 202 }, { $push: { deptwork: 11 } })

Unlike $addToSet, $push doesn't check for duplicates — pushing the same value twice adds it twice.

$push Modifiers: $each, $position, $slice, $sort

// add multiple values at once
db.employees.updateOne({ empid: 202 }, { $push: { deptwork: { $each: [3, 4] } } })

// insert at a specific index instead of the end
db.employees.updateOne(
  { empid: 202 },
  { $push: { deptwork: { $each: [10, 11], $position: 2 } } }
)

// keep only the last 4 elements after pushing
db.employees.updateOne(
  { empid: 204 },
  { $push: { deptwork: { $each: [10, 11, 12], $slice: -4 } } }
)

// push and re-sort the array (ascending or descending)
db.employees.updateOne(
  { empid: 204 },
  { $push: { deptwork: { $each: [30], $sort: 1 } } }
)

// $each: [] with $sort lets you re-sort WITHOUT pushing anything new
db.employees.updateOne(
  { empid: 202 },
  { $push: { deptwork: { $each: [], $sort: -1 } } }
)

Common Errors & Troubleshooting

• $pushing a value you meant to $addToSet — $push never checks for duplicates; if you actually wanted "add only if not already present," you need $addToSet, not $push.
• $[<identifier>] without a matching arrayFilters — the filtered positional operator requires a corresponding arrayFilters option in the same call; omitting it (or misspelling the identifier) throws an error.
• The positional $ operator matching the wrong document when the array condition also filters documents — $ always refers to the first array element that matched the query's array condition on that document, which can be surprising if the query condition and the update path don't line up as expected.

Key Takeaways

• $push allows duplicates; $addToSet never does.
• Use $each whenever you need to add or remove several array values in one call.
• The positional $ updates the first match; $[] updates all elements; $[<id>] + arrayFilters updates a filtered subset.