$ltrim (aggregation)

Definition

$ltrim

Removes whitespace characters, including null, or the specified characters from the beginning of a string.

$ltrim has the following syntax:

{ $ltrim: { input: <string>,  chars: <string> } }

The $ltrim takes a document with the following fields:

Field	Description
input	The string to trim. The argument can be any valid expression that resolves to a string. For more information on expressions, see Expression Operators.
chars	Optional. The character(s) to trim from the beginning of the input. The argument can be any valid expression that resolves to a string. The $ltrim operator breaks down the string into individual UTF code point to trim from input. If unspecified, $ltrim removes whitespace characters, including the null character. For the list of whitespace characters, see Whitespace Characters.

Tip

See also:

• $trim

• $rtrim

Behavior

• By default, $ltrim removes whitespace characters, including the null character, from the beginning of the input string:

  Example	Results
  { $ltrim: { input: " \n good bye \t " } }	"good bye \t "

• You can override the default characters to trim using the chars field.

  For example, the following trims any g and e from the start of the input string. Since the input starts with a whitespace, neither character can be trimmed from the start of the string.

  Example	Results
  { $ltrim: { input: " ggggoodbyeeeee", chars: "ge" } }	" ggggoodbyeeeee"

• If overriding the default characters to trim, you can explicitly include the whitespace character(s) to trim in the chars field.

  For example, the following trims any space, g, or d from the start of the input string.

  Example	Results
  { $ltrim: { input: " ggggoodbyeeeee ", chars: " gd" } }	"oodbyeeeee "

Example

Consider an inventory collection with the following documents:

{ "_id" : 1, "item" : "ABC1", quarter: "13Q1", "description" : " product 1" }
{ "_id" : 2, "item" : "ABC2", quarter: "13Q4", "description" : "product 2 \n The product is in stock.  \n\n  " }
{ "_id" : 3, "item" : "XYZ1", quarter: "14Q2", "description" : null }

The following operation uses the $ltrim operator to remove leading whitespaces from the description field:

db.inventory.aggregate([
   { $project: { item: 1, description: { $ltrim: { input: "$description" } } } }
])

The operation returns the following results:

{ "_id" : 1, "item" : "ABC1", "description" : "product 1" }
{ "_id" : 2, "item" : "ABC2", "description" : "product 2 \n The product is in stock.  \n\n  " }
{ "_id" : 3, "item" : "XYZ1", "description" : null }