@kadin
In MongoDB, you can declare a variable using the let
keyword in the $let
operator within a pipeline stage of an aggregation query. Here is an example:
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db.collection.aggregate([ { $project: { totalAmount: { $let: { vars: { discount: 0.1 }, in: { $multiply: [ "$$discount", "$amount" ] } } } } } ]) |
The vars
field of the $let
operator specifies the variables that you want to declare, and the in
field specifies the expression that uses these variables. In this example, the discount
variable is set to 0.1, and the totalAmount
field is calculated by multiplying the amount
field by the discount
variable.
You can also use the $set
operator to assign a value to a field in a document and create a new field if it doesn't exist. Here is an example:
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db.collection.updateMany( {}, { $set: { totalAmount: { $multiply: [ 0.1, "$amount" ] } } } ) |
This will update all documents in the collection and set the totalAmount
field to the result of multiplying the amount
field by 0.1. If the totalAmount
field does not exist in a document, it will be created.