[MongoDB Query: `$expr` operator] Mongodb query operator `$expr` usage #mongodb

use-expr-operator.js

// https://docs.mongodb.com/manual/reference/operator/query/expr/#op._S_expr
//// $expr can build query expressions that compare fields from the same document in a $match stage.

////// Compare Two Fields from A Single Document
////// The following operation uses $expr to find documents where the spent amount exceeds the budget
db.monthlyBudget.find( { $expr: { $gt: [ "$spent" , "$budget" ] } } )

////// Using $expr With Conditional Statements
////// The following operation uses $expr with $cond to mimic a conditional statement. It finds documents where the price is less than 5 after the applied discounts.
db.supplies.find( {
    $expr: {
       $lt:[ {
          $cond: {
             if: { $gte: ["$qty", 100] },
             then: { $divide: ["$price", 2] },
             else: { $divide: ["$price", 4] }
           }
       },
       5 ] }
} )