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Expressions are path or formulas to access and modify the data inside a document. Based on the concept of JSON path (, LiteDB supports a similar syntax to navigate inside a document.

BsonExpression is the class that parses a path expression and compiles it into a Linq Expression to be evaluated by LiteDB.

  • Path starts with $: $.Address.Street, where $ represents the root document. The $ symbol are optional and default in document navigation (Address.Street works too)
  • Int values are defined by [0-9]*: 123
  • Double values are defined by [0-9].[0-9]: 123.45
  • Strings are represented with a single/double quote: 'Hello World'
  • Null is represented by null
  • Bool is represented using true or false keywords.
  • Document starts with { key1: <value|expression>, key2: ... }
  • Arrays are represented with [<value|expression>, <value|expression>, ...]
  • Functions are represented with FUNCTION_NAME(par1, par2, ...): LOWER($.Name)


  • $.Price
  • $.Price + 100
  • SUM($.Items[*].Price)
var expr = new BsonExpression("SUM($.Items[*].Unity * $.Items[*].Price)");
var total = expr.Execute(doc, true).First().AsDecimal;

Expressions can be used in many ways:

  • Creating an index based on an expression:
    • collection.EnsureIndex("idx_name", "LOWER($.Name)", false)
    • collection.EnsureIndex(x => x.Name.ToLower())
  • Querying documents inside a collection based on expression (full scan search)
    • collection.Find("SUBSTRING($.Name, 0, 1) = 'T'")
  • Update using SQL syntax
    • UPDATE customers SET Name = LOWER($.Name) WHERE _id = 1
  • Creating new document result in SELECT shell command
    • SELECT { upper_titles: ARRAY(UPPER($.Books[*].Title)) } WHERE $.Name LIKE "John%"
  • Querying documents using the SQL syntax
    • SELECT $.Name, $.Phones[@.Type = "Mobile"] FROM customers


  • $ - Root
  • $.Name - Name field
  • $.Name.First - First field from Name subdocument
  • $.Books - Returns an array of books
  • $.Books[0] - Returns the first book inside Books array
  • $.Books[*] - Returns every book inside Books
  • $.Books[*].Title Returns the title from every book in Books
  • $.Books[-1] - Returns the last book inside Books array

Path also supports expressions to filter child nodes

  • $.Books[@.Title = 'John Doe'] - Returns all books where Title is 'John Doe'

  • $.Books[@.Price > 100].Title - Returns all titles where Price is greater than 100

Inside an array, @ acts as a sub-iterator, pointing to the current sub-document. It’s possible use functions inside expressions too:

  • $.Books[SUBSTRING(LOWER(@.Title), 0, 1) = 't'] - Returns all books whose Title starts with 'T' or 't'.

Difference between $ and *

In SQL query (at SELECT segment) is possible use both $ and *. But they represent different things.

  • $ represent current document root. When use $ (or ommited, because this symbol are optional and are default) you are referencing about root current document.

  • * represent all grouped documents. It’s not more about a single document but all documents in group (or in query result). Used when GROUP BY are present or when you want return a single value in query (like SELECT COUNT(*) FROM customers).

If you use SELECT $ FROM customers you will get IEnumerable<BsonDocument> result (N documents). If you use SELECT * FROM customers you will get a single value, a BsonArray with all documents result inside. Be carful because if your resultset are big you are creating a very large single array in memory.


Functions are used to manipulate data in expressions. A few examples will be provided for each category of functions. For a complete list of functions, check the API documentation.

Aggregate Functions

Aggregate functions take an array as input and return a single value.

  • COUNT(arr) - Returns the number of elements in the array arr
  • AVG(arr) - Returns the average value in the array arr
  • LAST(arr) - Returns the last element in the array arr

DataType Functions

DataType functions provide explicit data type conversion.

  • STRING(expr) - Returns the result of expr converted to string
  • INT32(expr) - Tries to convert the result of expr to an Int32, returning null if not possible
  • DATETIME(expr) - Tries to convert the result of expr to a DateTime, returning null if not possible

Date Functions

  • YEAR(date) - Returns the year value from date
  • DATEADD('year', 3, date) - Returns a new date with 3 years added to date
  • DATEDIFF('day', dateStart, dateEnd) - Returns the difference in days between dateEnd and dateStart

Math Functions

  • ABS(num) - Returns the absolute value of num
  • ROUND(num, digits) - Returns num rounded to digits digits
  • POW(base, exp) - Returns base to the power of exp

String Functions

  • UPPER(str) - Returns str in uppercase
  • TRIM(str) - Returns a new string without leading and trailing white spaces
  • REPLACE(str, old, new) - Returns a new string with every ocurrence of old in str replaced by new

High-Order Functions

High-Order functions take an array and a lambda expression that is applied to every document in the array. Use @ symbol to represent inner looped value.

  • MAP(arr => expr) returns a new array with the map expression applied to each element

    • MAP([1,2,3] => @*2) returns [2,4,6]
    • MAP([{a:1, b:2}, {a:3, b:4}] => @.a) returns [1,3]
  • FILTER(arr => expr) returns a new array containing only the elements for which the filter expression returns true

    • FILTER([1,2,3,4,5] => @ > 3) returns [4,5]
    • FILTER([{a:1, b:2}, {a:2}] => @.b != null) returns [{a:1, b:2}]
  • SORT(arr => expr) returns a new array sorted by the result of expr in ascending order -SORT([3,2,5,1,4] => @) returns [1,2,3,4,5] -SORT([{a:2}, {a:1, b:2}] => @.a) returns [{a:1, b:2}, {a:2}]

  • SORT(arr => expr, order) returns a new array sorted by the result of expr with the order defined by order (ascending if order is 1 or 'asc', descending if order is -1 or 'desc') -SORT([3,2,5,1,4] => @, 'desc') returns [5,4,3,2,1] -SORT([{a:1, b:2}, {a:2}] => @.a, -1) returns [{a:2}, {a:1, b:2}]

Misc Functions

  • JSON(str) - Takes a string representation of a JSON and returns a parsed BsonValue containing the document
  • CONCAT(arr1, arr2) - Returns a new array containg the concatenation between arrays arr1 and arr2
  • RANDOM(min, max) - Returns a random Int32 between min and max