Aggregation in Mongo DB

aggregation.md

📘 MongoDB Basic Aggregation – Full Guide (Markdown)

✅ BASE SETUP (Express + Mongoose)

User Model

const UserSchema = new mongoose.Schema({
  name: String,
  age: Number,
  email: String,
  isActive: Boolean,
  city: String
});

const User = mongoose.model("User", UserSchema);

---

🚀 API Route Structure

app.get("/api/users/basic", async (req, res) => {
  try {
    const result = await User.aggregate([
      // aggregation stages here
    ]);

    res.json(result);
  } catch (err) {
    res.status(500).json({ error: err.message });
  }
});

---

🟩 1️⃣ $match — Filter Documents

Syntax

{ 
  $match: { field: value } 
}

Common Use Cases

Purpose	Example
Equal match	{ age: 25 }
Greater than	{ age: { $gt: 20 } }
AND condition	{ age: { $gt: 18 }, isActive: true }
OR condition	{ $or: [{ city: "Hyd" }, { age: { $lt: 18 } }] }

Example

{ $match: { isActive: true } }

Output

[
  { "name": "Suhail", "isActive": true },
  { "name": "Ali", "isActive": true }
]

---

🟦 2️⃣ $project — Select / Hide Fields

Syntax

{
  $project: {
    field1: 1,
    field2: 0,
    newField: "$oldField"
  }
}

⚠️ Rules

• You cannot mix 1 and 0 in the same projection except _id: 0.

Example

{ 
  $project: { 
    name: 1,
    age: 1,
    _id: 0
  } 
}

Output

[
  { "name": "Suhail", "age": 23 },
  { "name": "Ali", "age": 27 }
]

---

🟨 3️⃣ $addFields — Add New or Modify Fields

Syntax

{
  $addFields: {
    newField: expression
  }
}

Common Expressions

Expression	Meaning
{ $toUpper: "$name" }	Convert string to uppercase
{ $concat: ["Hello ", "$name"] }	Concatenate
{ $sum: ["$field1", "$field2"] }	Add values
{ $multiply: ["$price", 2] }	Multiply

Example

{
  $addFields: {
    nameUpper: { $toUpper: "$name" },
    isAdult: { $gt: ["$age", 17] }
  }
}

Output

[
  { "name": "Suhail", "age": 23, "nameUpper": "SUHAIL", "isAdult": true }
]

---

🟥 4️⃣ $sort — Sort Documents

Syntax

{
  $sort: { field: 1 or -1 }
}

Values

Value	Meaning
1	Ascending
-1	Descending

Example

{ $sort: { age: -1 } }

Output

[
  { "name": "Ali", "age": 30 },
  { "name": "Suhail", "age": 23 }
]

---

🟫 5️⃣ $skip — Skip N Documents

Syntax

{ $skip: number }

Example

{ $skip: 5 }

---

🟧 6️⃣ $limit — Limit Number of Documents

Syntax

{ $limit: number }

Example

{ $limit: 10 }

---

🟩 7️⃣ $count — Count Documents

Syntax

{ $count: "totalUsers" }

Example

{ $match: { isActive: true } },
{ $count: "activeUsers" }

Output

[
  { "activeUsers": 42 }
]

---

🚀 FULL EXAMPLE — All Basic Stages Combined

app.get("/api/users/basic", async (req, res) => {
  try {
    const result = await User.aggregate([

      // 1️⃣ Filter users
      { $match: { isActive: true } },

      // 2️⃣ Add computed fields
      { 
        $addFields: { 
          nameUpper: { $toUpper: "$name" }
        }
      },

      // 3️⃣ Select fields
      { 
        $project: { 
          name: 1,
          age: 1,
          nameUpper: 1,
          email: 1,
          _id: 0
        }
      },

      // 4️⃣ Sort by age (descending)
      { $sort: { age: -1 } },

      // 5️⃣ Skip 0 docs
      { $skip: 0 },

      // 6️⃣ Limit results to 10
      { $limit: 10 }

      // 7️⃣ Count docs (use separate route)
      // { $count: "total" }
    ]);

    res.json(result);
  } catch (err) {
    res.status(500).json({ error: err.message });
  }
});

---

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