How to Add Element to Array in MongoDB: Simple Guide

To add an element to an array in MongoDB, use the $push update operator with updateOne or updateMany. This operator appends the specified value to the array field in the document.

Syntax

The $push operator adds a value to an array field in a document. It is used with update commands like updateOne or updateMany.

• filter: Selects the document(s) to update.
• update: Uses { $push: { arrayField: value } } to add value to arrayField.

mongodb

db.collection.updateOne(
  { <filter> },
  { $push: { <arrayField>: <value> } }
)

Example

This example adds the string "apple" to the fruits array of the document where name is "John".

mongodb

db.users.updateOne(
  { name: "John" },
  { $push: { fruits: "apple" } }
)

Output

{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }

Common Pitfalls

1. Using $push on a non-array field: This causes an error because $push only works on arrays.

2. Forgetting the filter: Without a proper filter, you might update the wrong documents or none at all.

3. Using $push instead of $addToSet when you want to avoid duplicates: $push always adds the element, even if it already exists.

mongodb

/* Wrong: pushing to a non-array field */
db.users.updateOne(
  { name: "John" },
  { $push: { age: 30 } }  // 'age' is not an array
)

/* Right: push to an array field */
db.users.updateOne(
  { name: "John" },
  { $push: { fruits: "banana" } }
)

Quick Reference

• $push: Adds an element to an array.
• $addToSet: Adds an element only if it does not exist (no duplicates).
• updateOne(filter, update): Updates one matching document.
• updateMany(filter, update): Updates all matching documents.

Key Takeaways

Use the $push operator to add elements to arrays in MongoDB documents.

Always specify a filter to target the correct document(s) when updating.

Ensure the target field is an array before using $push to avoid errors.

Use $addToSet instead of $push to prevent duplicate entries in arrays.