Count the number of active users per tenant. Filter users with `status: "active"`, group by `tenantId`, and sort by count descending.

Problem Statement

<p>Count the number of active <code>users</code> per tenant. Filter <code>users</code> with <code>status: "active"</code>, group by <code>tenantId</code>, and sort by count descending.</p>

Examples

Input: users collection: +-----+----------+----------+ | _id | tenantId | status | +-----+----------+----------+ | 1 | t1 | active | | 2 | t1 | inactive | | 3 | t2 | active | | 4 | t2 | active | | 5 | t1 | active | +-----+----------+----------+

Output: +-----+-------------+ | _id | activeCount | +-----+-------------+ | t1 | 2 | | t2 | 2 | +-----+-------------+

Explanation: The query retrieves all documents from the collection that satisfy the specified filter criteria.

Complexity

Time Complexity: -

Space Complexity: -

Hints

💡 Hint 1: This problem requires grouping documents by a key and aggregating values. Use db.<collection>.aggregate([...]) with a $group stage. 💡 Hint 2: In the $group stage, set _id to the grouping field (e.g. "$category" or "$dept") and use accumulator operators like $sum, $avg, $max, or $min. 💡 Hint 3: Remember to call .toArray() at the end of the aggregation pipeline to return the resolved documents array.

Editorial & Approach

Problem Overview & Intuition

To solve "Multi-Tenant Active Users Per Tenant", we query the MongoDB document store. The goal is to count the number of active users per tenant. filter users with `status: "active"`, group by `tenantid`, and sort by count descending. Using an aggregation pipeline, the database engine filters and structures the BSON documents efficiently.

Step-by-Step Approach

Identify Target Collection: Access the collection through the db instance.
Construct Query / Pipeline: Build the aggregation stages ($match, $group, $sort, etc.).
Resolve Cursor: Invoke .toArray() to transform the query cursor into the required array of documents.

Optimal Implementation (MongoDB)

function solve(db) {
  return db.users.aggregate([
    { $match: { status: "active" } },
    { $group: { _id: "$tenantId", activeCount: { $sum: 1 } } },
    { $sort: { activeCount: -1 } }
  ]);
}

Complexity Analysis

Time Complexity O(N) pipeline traversal through aggregation stages.

Space Complexity O(M) intermediate document buffer in aggregation pipeline.

Key Considerations & Edge Cases

Empty Collections: If no documents match, the query cleanly returns an empty array [].
Missing / NULL Fields: Missing fields in documents are handled safely without throwing runtime exceptions.
Type Coercion: BSON types (ObjectId, Numbers, Strings) are compared strictly according to MongoDB specifications.

Multi-Tenant Active Users Per Tenant

Hard

Count the number of active users per tenant. Filter users with status: "active", group by tenantId, and sort by count descending.

Example Scenarios

1Example 1

Input:

users collection

_id	tenantId	status
1	t1	active
2	t1	inactive
3	t2	active
4	t2	active
5	t1	active

Output:

_id	activeCount
t1	2
t2	2

Explanation:

The query retrieves all documents from the collection that satisfy the specified filter criteria.

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Query Results (JSON)

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