$eq

The $eq (equal) operator in MongoDB is used for comparison operations. It compares two values, and if they are equal, the result is true. Otherwise, the result is false.

The $eq operator can be used in queries to filter documents based on a specific field's value. It can also be used in aggregations where you can determine whether two fields' values or expressions are equal.

Usage

In a query, the $eq operator can be used as follows:

db.collection.find({ field: { $eq: value } });

For example, if you have a collection named products and you want to find all documents where the price field is equal to 100, you can use the $eq operator like this:

db.products.find({ price: { $eq: 100 } });

Usage in Aggregations

In an aggregation pipeline, the $eq operator can be used within the $project, $match, $addFields, and other stages with expressions. For example, if you want to add a field "discounted" to the documents based on whether the price field is equal to 50, you can use the $eq operator like this:

db.products.aggregate([
  {
    $addFields: {
      discounted: {
        $eq: ['$price', 50],
      },
    },
  },
]);

This will add a new field named "discounted" with a true or false value based on whether the price field is equal to 50.

In conclusion, the $eq operator is a helpful tool in MongoDB for performing equality checks and filtering documents based on matching values in queries and aggregations.