Delete all expired coupons (`expired: true`), insert a new coupon `{ _id: 10, code: "NEW50", expired: false }`, and return the total count of coupons.

Problem Statement

<p>Delete all expired coupons (<code>expired: true</code>), insert a new coupon <code>{ <code>_id</code>: 10, code: "NEW50", expired: false }</code>, and return the total count of coupons.</p>

Examples

Output: +-------+ | count | +-------+ | 3 | +-------+

Explanation: The query calculates the total count of documents that satisfy the given filter criteria.

Complexity

Time Complexity: -

Space Complexity: -

Hints

💡 Hint 1: Use db.<collection>.find(<filter>) to query the documents in the collection. 💡 Hint 2: Pass an empty object {} to retrieve all documents, or specify key-value pairs for exact matching. 💡 Hint 3: Chain .toArray() to resolve the MongoDB cursor into a JavaScript array: return db.<collection>.find(...).toArray();

Editorial & Approach

Problem Overview & Intuition

To solve "Multi-Step: Delete + Insert + Count", we query the MongoDB document store. The goal is to delete all expired coupons (`expired: true`), insert a new coupon `{ _id: 10, code: "new50", expired: false }`, and return the total count of coupons. Using a targeted find query, 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 query filter with appropriate comparison operators.
Resolve Cursor: Invoke .toArray() to transform the query cursor into the required array of documents.

Optimal Implementation (MongoDB)

function solve(db) {
  db.coupons.deleteMany({ expired: true });
  db.coupons.insertOne({ _id: 10, code: "NEW50", expired: false });
  return db.coupons.countDocuments({});
}

Complexity Analysis

Time Complexity O(N) collection scan (O(log N) if index is present on filtered fields).

Space Complexity O(K) where K is the number of returned documents in memory.

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-Step: Delete + Insert + Count

Medium

Delete all expired coupons (expired: true), insert a new coupon { _id: 10, code: "NEW50", expired: false }, and return the total count of coupons.

Example Scenarios

1Example 1

Input:

coupons collection

_id	code	expired
1	SAVE10	true
2	SAVE20	false
3	SAVE30	true
4	SAVE40	false

Output:

count
3

Explanation:

The query calculates the total count of documents that satisfy the given filter criteria.

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Query Results (JSON)

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