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🔤 MongoDB: Type Conversion and Checking Operators

MongoDB provides several type conversion and type checking operators to transform field data types and verify data formats inside aggregation pipelines. These operators are extremely useful when working with inconsistent or mixed-type data.

📚 Sample Collection Setup

Example01
MongoDB Shell
NoSQL
Database
1db.studentsArr.insertMany([
2  { _id: 1, name: "Alice", scores: [85, 92, 88, 76, 95], tags: ["js", "py", "mongo"] },
3  { _id: 2, name: "Bob",   scores: [70, 65, 88],         tags: ["java", "c#"] },
4  { _id: 3, name: "Cara",  scores: [100, 99, 98],        tags: ["go", "rust"] }
5]);
Query Result
JSON Output
✅ 3 student documents inserted successfully.

1️⃣ Using $toString — Convert to String

Example02
MongoDB Shell
NoSQL
Database
1db.studentsArr.aggregate([
2  { $project: {
3      name: 1,
4      scoresString: { $toString: "$scores" } // Converts array to string
5  }}
6]);
Query Result
JSON Output
{ "_id": 1, "name": "Alice", "scoresString": "[85, 92, 88, 76, 95]" }

2️⃣ Using $toInt — Convert to Integer

Example03
MongoDB Shell
NoSQL
Database
1db.studentsArr.aggregate([
2  { $project: {
3      name: 1,
4      firstScoreInt: { $toInt: { $arrayElemAt: ["$scores", 0] } }
5  }}
6]);
Query Result
JSON Output
{ "_id": 1, "name": "Alice", "firstScoreInt": 85 }

3️⃣ Using $toLong — Convert to 64-bit Integer

Example04
MongoDB Shell
NoSQL
Database
1db.studentsArr.aggregate([
2  { $project: {
3      name: 1,
4      totalAsLong: { $toLong: { $sum: "$scores" } }
5  }}
6]);
Query Result
JSON Output
{ "_id": 1, "name": "Alice", "totalAsLong": NumberLong(436) }

4️⃣ Using $toDouble — Convert to Floating-Point

Example05
MongoDB Shell
NoSQL
Database
1db.studentsArr.aggregate([
2  { $project: {
3      name: 1,
4      avgAsDouble: { $toDouble: { $avg: "$scores" } }
5  }}
6]);
Query Result
JSON Output
{ "_id": 1, "name": "Alice", "avgAsDouble": 87.2 }

5️⃣ Using $toDecimal — Convert to Decimal128

Example06
MongoDB Shell
NoSQL
Database
1db.studentsArr.aggregate([
2  { $project: {
3      name: 1,
4      preciseValue: { $toDecimal: { $multiply: ["$scores.0", 1.1] } }
5  }}
6]);
Query Result
JSON Output
{ "_id": 1, "name": "Alice", "preciseValue": NumberDecimal("93.5") }

6️⃣ Using $toBool — Convert to Boolean

Example07
MongoDB Shell
NoSQL
Database
1db.studentsArr.aggregate([
2  { $project: {
3      name: 1,
4      hasScores: { $toBool: "$scores" }
5  }}
6]);
Query Result
JSON Output
{ "_id": 1, "name": "Alice", "hasScores": true }

7️⃣ Using $toDate — Convert to Date

Example08
MongoDB Shell
NoSQL
Database
1db.dates.aggregate([
2  { $project: {
3      eventDate: { $toDate: "$dateStr" }
4  }}
5]);
Query Result
JSON Output
{ "eventDate": ISODate("2023-01-01T00:00:00Z") }

8️⃣ Using $toObjectId — Convert to ObjectId

Example09
MongoDB Shell
NoSQL
Database
1db.users.aggregate([
2  { $project: {
3      userId: { $toObjectId: "$hexId" }
4  }}
5]);
Query Result
JSON Output
✅ Converts valid 12-byte hex string to ObjectId.

9️⃣ Using $convert — Safe Type Conversion

Example10
MongoDB Shell
NoSQL
Database
1db.studentsArr.aggregate([
2  { $project: {
3      name: 1,
4      safeConversion: {
5        $convert: {
6          input: { $arrayElemAt: ["$tags", 0] },
7          to: "int",
8          onError: "Invalid",
9          onNull: null
10        }
11      }
12  }}
13]);
Query Result
JSON Output
{ "_id": 1, "name": "Alice", "safeConversion": "Invalid" }

🔟 Using $type — Check BSON Type

Example11
MongoDB Shell
NoSQL
Database
1db.studentsArr.aggregate([
2  { $project: {
3      name: 1,
4      scoresType: { $type: "$scores" }
5  }}
6]);
Query Result
JSON Output
{ "_id": 1, "name": "Alice", "scoresType": "array" }

1️⃣1️⃣ Using $isNumber — Numeric Check

Example12
MongoDB Shell
NoSQL
Database
1db.studentsArr.aggregate([
2  { $project: {
3      name: 1,
4      isNumeric: { $isNumber: { $avg: "$scores" } }
5  }}
6]);
Query Result
JSON Output
{ "_id": 1, "name": "Alice", "isNumeric": true }