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$toLower

The $toLower operator converts a string to all lowercase letters.

📌 Syntax

{ "$toLower": <expression> }

The expression must resolve to a string.

✅ Base Example 1 – Lowercase Email

📥 Input Document

{ "email": "USER@EXAMPLE.COM" }

📌 Expression

{ "$toLower": "$email" }

📤 Output

"user@example.com"

✅ Base Example 2 – City Name Normalization

📥 Input Document

{ "city": "Toronto" }

📌 Expression

{ "$toLower": "$city" }

📤 Output

"toronto"

🧱 Ecommerce Example – Normalize Brand Name

📌 Pipeline

[
  { "$unwind": "$items" },
  {
    "$project": {
      "brand_normalized": {
        "$toLower": "$items.brand"
      },
      "product": "$items.name"
    }
  }
]

📥 Input Document

{
  "items": [
    { "name": "Laptop", "brand": "HP" },
    { "name": "Keyboard", "brand": "LogiTech" }
  ]
}

📤 Output

[
  { "brand_normalized": "hp", "product": "Laptop" },
  { "brand_normalized": "logitech", "product": "Keyboard" }
]

🔧 Common Use Cases

  • Case-insensitive comparisons
  • Normalization for search, filter, and indexing
  • String unification in user inputs
  • $toUpper, $trim, $concat, $substr, $toString

🧠 Notes

  • If the input is not a string, it will raise an error.
  • Use $ifNull or $cond to handle missing values gracefully.