Filter in MongoDB

What is $filter in MongoDB?

Filter in MongoDB is a powerful aggregation pipeline stage that allows you to filter the elements of an array field within a document. It enables you to specify a condition or criteria that determines which array elements should be included or excluded from the result. The $filter operator takes an input array and a boolean expression as parameters. It then returns a new array that only contains the elements for which the expression evaluates to true.

Syntax:

The syntax of the $filter operator is as follows:

Parameters

Here's a breakdown of the parameters:

• input: Specifies the array field you want to filter.
• as: Defines a variable that represents each element of the array during the filtering process.
• cond: Defines the boolean expression or condition that determines which elements should be included in the result. It can reference the variable defined by as.

Fields and Specifications

When using the filter in MongoDB, the fields and specifications refer to the parameters used within the $filter stage of the aggregation pipeline. Let's break down the fields and specifications specific to $filter:

Behavior

The filter in the MongoDB aggregation framework behaves as follows:

• Takes an input array and a condition as parameters.
• Applies the condition to each element of the input array.
• Creates a new array containing only the elements that satisfy the condition.
• Maintains the order of the elements in the input array.
• Uses variable binding to refer to the current element during condition evaluation.
• Returns an empty array if the input array is empty or evaluates to null.

The $filter operator enables selective filtering of array elements based on a condition, allowing for flexible data manipulation within MongoDB's aggregation pipeline.

How Filter in MongoDB Works?

The Filter operator in the MongoDB aggregation framework works by applying a filtering condition to each element of an input array and creating a new array containing only the elements that satisfy the condition. Here's how the filter operation works:

1. Input Array: The $filter operator takes an input array as its first parameter. This array can be any valid expression that resolves to an array, such as a field value in the document or the result of a previous pipeline stage.
2. Condition: The $filter operator applies a filtering condition to each element of the input array. This condition is specified using the cond field and can include comparison operators, logical operators, and values. The condition determines whether an element should be included in the output array.
3. Variable Binding: Within the condition, you can refer to the current element being evaluated using a variable defined by the as field. This allows you to perform comparisons or apply expressions based on the element's value.
4. Evaluation: The $filter operator iterates over each element of the input array and evaluates the specified condition for each element. If the condition evaluates to true for an element, that element is included in the output array. If the condition evaluates to false, the element is excluded.
5. Output Array: The $filter operator returns a new array containing only the elements that satisfy the specified condition. The order of the elements in the output array corresponds to their order in the input array.
6. Empty Array Handling: If the input array is empty or evaluates to null, the $filter operator returns an empty array as the result.

Examples of Filter in MongoDB

Here are a few examples to demonstrate the usage of the $filter operator in MongoDB's aggregation framework:

Example 1: Filtering array elements based on a condition:

Output:

Explanation: In this example, the $filter operator is used to filter the elements of the myArray field. It keeps only the elements that are greater than 5.

Example 2: Filtering and transforming array elements:

Output:

Explanation: Here, the $filter operator filters the elements of myArray, but with an additional condition. It keeps only the elements that are greater than 5 and less than 10.

Example 3: Filtering based on nested fields:

Output:

Explanation: In this example, the $filter operator is applied to the myArray field, but the condition checks if the nestedField within each element is equal to "value". It keeps only the elements that satisfy this condition.