How to Use $eq Operator in MongoDB Queries

In MongoDB, the $eq operator is used to match documents where a field's value is exactly equal to a specified value. It is commonly used inside a query object like { field: { $eq: value } } to filter results by equality.

Syntax

The $eq operator checks if a field's value is equal to the given value.

• field: The name of the field to compare.
• $eq: The equality operator.
• value: The value to compare against.

It is used inside a query object like this:

json

{ field: { $eq: value } }

Example

This example finds all documents in the users collection where the age field is exactly 25.

mongodb

db.users.find({ age: { $eq: 25 } })

Output

[
  { "_id": ObjectId("..."), "name": "Alice", "age": 25 },
  { "_id": ObjectId("..."), "name": "Bob", "age": 25 }
]

Common Pitfalls

One common mistake is using $eq unnecessarily when a simple equality check works the same. For example, { age: 25 } is equivalent to { age: { $eq: 25 } }.

Another pitfall is confusing $eq with assignment or using it outside query context.

mongodb

/* Wrong: Using $eq outside query */
db.users.insertOne({ name: "Eve", age: { $eq: 30 } })

/* Right: Use $eq only in queries */
db.users.find({ age: { $eq: 30 } })

Quick Reference

Use $eq to test equality in queries. It is optional when testing simple equality because MongoDB treats { field: value } as { field: { $eq: value } }.

Use $eq when you want to be explicit or combine with other operators.

Operator	Description	Example
$eq	Matches values that are equal to a specified value	{ age: { $eq: 25 } }
Simple equality	Shorthand for $eq	{ age: 25 }

Key Takeaways

Use $eq to explicitly match documents where a field equals a value.

Simple equality queries like { field: value } work the same as { field: { $eq: value } }.

Do not use $eq outside query filters, such as in insert or update documents.

Use $eq when combining with other operators for clarity.

Remember $eq is part of MongoDB's query operators for filtering data.