$let (aggregation)
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Definition
$let
Binds variables for use in the specified expression, and returns the result of the expression.
The
$let
expression has the following syntax:{ $let: { vars: { <var1>: <expression>, ... }, in: <expression> } } FieldSpecificationvars
Assignment block for the variables accessible in the
in
expression. To assign a variable, specify a string for the variable name and assign a valid expression for the value.The variable assignments have no meaning outside the
in
expression, not even within thevars
block itself.in
The expression to evaluate.To access variables in aggregation expressions, prefix the variable name with double dollar signs (
$$
) and enclose in quotes. For more information on expressions, see Expressions. For information on use of variables in the aggregation pipeline, see Variables in Aggregation Expressions.
Behavior
$let
can access variables defined outside its expression
block, including system variables.
If you modify the values of externally defined variables in the
vars
block, the new values take effect only in the in
expression. Outside of the in
expression, the variables retain
their previous values.
In the vars
assignment block, the order of the assignment does
not matter, and the variable assignments only have meaning inside
the in
expression. As such, accessing a variable's value in the
vars
assignment block refers to the value of the variable defined
outside the vars
block and not inside the same vars
block.
For example, consider the following $let
expression:
{ $let: { vars: { low: 1, high: "$$low" }, in: { $gt: [ "$$low", "$$high" ] } } }
In the vars
assignment block, "$$low"
refers to the value of an
externally defined variable low
and not the variable defined in the
same vars
block. If low
is not defined outside this
$let
expression block, the expression is invalid.
Example
A sales
collection has the following documents:
{ _id: 1, price: 10, tax: 0.50, applyDiscount: true } { _id: 2, price: 10, tax: 0.25, applyDiscount: false }
The following aggregation uses $let
in the
$project
pipeline stage to calculate and return the
finalTotal
for each document:
db.sales.aggregate( [ { $project: { finalTotal: { $let: { vars: { total: { $add: [ '$price', '$tax' ] }, discounted: { $cond: { if: '$applyDiscount', then: 0.9, else: 1 } } }, in: { $multiply: [ "$$total", "$$discounted" ] } } } } } ] )
The aggregation returns the following results:
{ "_id" : 1, "finalTotal" : 9.450000000000001 } { "_id" : 2, "finalTotal" : 10.25 }