MongoDB Query to Pull Elements from Array

Use the $pull operator in MongoDB to remove elements from an array that match a specified condition, for example: db.collection.updateOne({ _id: 1 }, { $pull: { tags: 'red' } }) removes 'red' from the tags array.

Examples

Input{ _id: 1, tags: ['red', 'blue', 'green'] }, query: { $pull: { tags: 'red' } }

Output{ _id: 1, tags: ['blue', 'green'] }

Input{ _id: 2, scores: [10, 20, 30, 20] }, query: { $pull: { scores: 20 } }

Output{ _id: 2, scores: [10, 30] }

Input{ _id: 3, items: ['apple', 'banana', 'cherry'] }, query: { $pull: { items: { $in: ['banana', 'cherry'] } } }

Output{ _id: 3, items: ['apple'] }

How to Think About It

To pull elements from an array in MongoDB, think about specifying which elements you want to remove by using the $pull operator with a condition. MongoDB will then find all matching elements in the array and remove them.

Algorithm

1

Identify the document(s) to update using a filter.

2

Specify the array field and the condition for elements to remove using <code>$pull</code>.

3

Run the update operation to remove matching elements from the array.

4

Verify the array no longer contains the pulled elements.

Code

mongodb

db.products.insertOne({ _id: 1, tags: ['red', 'blue', 'green'] });
db.products.updateOne(
  { _id: 1 },
  { $pull: { tags: 'red' } }
);
printjson(db.products.findOne({ _id: 1 }));

Output

{
  "_id" : 1,
  "tags" : [ "blue", "green" ]
}

Dry Run

Let's trace pulling 'red' from the tags array in the document with _id 1.

1

Initial Document

{ _id: 1, tags: ['red', 'blue', 'green'] }

2

Apply $pull Operator

Remove 'red' from tags array

3

Resulting Document

{ _id: 1, tags: ['blue', 'green'] }

Step	Array Before	Element Pulled	Array After
1	['red', 'blue', 'green']	'red'	['blue', 'green']

Why This Works

Step 1: Use $pull to Remove Elements

The $pull operator removes all array elements that match the given condition.

Step 2: Specify Condition

You provide the value or query condition inside $pull to target specific elements.

Step 3: Update Operation

MongoDB updates the document by removing matching elements from the array in place.

Alternative Approaches

Using $pull with $in operator

mongodb

db.collection.updateOne({ _id: 3 }, { $pull: { items: { $in: ['banana', 'cherry'] } } });

Removes multiple specified elements from the array in one operation.

Using $set with $filter aggregation (MongoDB 4.2+)

mongodb

db.collection.updateOne(
  { _id: 1 },
  [{ $set: { tags: { $filter: { input: '$tags', cond: { $ne: ['$$this', 'red'] } } } } }]
);

Filters array elements by condition, useful for complex filtering but less straightforward than $pull.

Complexity: O(n) time, O(1) space

Time Complexity

The operation scans the array elements once to find matches, so it is linear in the size of the array.

Space Complexity

The update happens in place without extra space proportional to the array size.

Which Approach is Fastest?

$pull is efficient for simple removals; using $filter with aggregation can be slower but more flexible.

Approach	Time	Space	Best For
$pull	O(n)	O(1)	Simple element removal by value or condition
$pull with $in	O(n)	O(1)	Removing multiple specific elements
$set with $filter	O(n)	O(1)	Complex filtering conditions on arrays