$expr

$expr in $lookup Operations

When $expr appears in a $match stage that is part of a $lookup subpipeline, $expr can refer to let variables defined by the $lookup stage. For an example, see Use Multiple Join Conditions and a Correlated Subquery.

The $eq, $lt, $lte, $gt, and $gte comparison operators placed in an $expr operator can use an index on the from collection referenced in a $lookup stage. Limitations:

• Indexes can only be used for comparisons between fields and constants, so the let operand must resolve to a constant.

  For example, a comparison between $a and a constant value can use an index, but a comparison between $a and $b cannot.

• Indexes are not used for comparisons where the let operand resolves to an empty or missing value.

• Multikey indexes are not used.

Compare Two Fields from a Single Document

$expr can contain expressions that compare fields from the same document.

Create a monthlyBudget collection with these documents:

db.monthlyBudget.insertMany( [
   { _id : 1, category : "food", budget : 400, spent : 450 },
   { _id : 2, category : "drinks", budget : 100, spent : 150 },
   { _id : 3, category : "clothes", budget : 100, spent : 50 },
   { _id : 4, category : "misc", budget : 500, spent : 300 },
   { _id : 5, category : "travel", budget : 200, spent : 650 }
] )

The following operation uses $expr to find documents where the spent amount exceeds the budget:

db.monthlyBudget.find( { $expr: { $gt: [ "$spent" , "$budget" ] } } )

Output:

{ _id : 1, category : "food", budget : 400, spent : 450 }
{ _id : 2, category : "drinks", budget : 100, spent : 150 }
{ _id : 5, category : "travel", budget : 200, spent : 650 }

Use $expr With Conditional Statements

Some queries require the ability to execute conditional logic when defining a query filter. The aggregation pipeline provides the $cond operator to express conditional statements. By using $expr with the $cond operator, you can specify a conditional filter for your query statement.

Create a sample supplies collection with the following documents:

db.supplies.insertMany( [
   { _id : 1, item : "binder", qty : NumberInt("100"), price : NumberDecimal("12") },
   { _id : 2, item : "notebook", qty : NumberInt("200"), price : NumberDecimal("8") },
   { _id : 3, item : "pencil", qty : NumberInt("50"), price : NumberDecimal("6") },
   { _id : 4, item : "eraser", qty : NumberInt("150"), price : NumberDecimal("3") },
   { _id : 5, item : "legal pad", qty : NumberInt("42"), price : NumberDecimal("10") }
] )

Assume that for an upcoming sale next month, you want to discount the prices such that:

• If qty is greater than or equal to 100, the discounted price will be 0.5 of the price.

• If qty is less than 100, the discounted price is 0.75 of the price.

Before applying the discounts, you would like to know which items in the supplies collection have a discounted price of less than 5.

The following example uses $expr with $cond to calculate the discounted price based on the qty and $lt to return documents whose calculated discount price is less than NumberDecimal("5"):

// Aggregation expression to calculate discounted price
let discountedPrice = {
   $cond: {
      if: { $gte: ["$qty", 100] },
      then: { $multiply: ["$price", NumberDecimal("0.50")] },
      else: { $multiply: ["$price", NumberDecimal("0.75")] }
   }
};
// Query the supplies collection using the aggregation expression
db.supplies.find( { $expr: { $lt:[ discountedPrice,  NumberDecimal("5") ] } });

The following table shows the discounted price for each document and whether discounted price is less than NumberDecimal("5") (i.e. whether the document meets the query condition).

The db.collection.find() operation returns the documents whose calculated discount price is less than NumberDecimal("5"):

{ _id : 2, item : "notebook", qty : 200 , price : NumberDecimal("8") }
{ _id : 3, item : "pencil", qty : 50 , price : NumberDecimal("6") }
{ _id : 4, item : "eraser", qty : 150 , price : NumberDecimal("3") }

Even though $cond calculates an effective discounted price, that price is not reflected in the returned documents. Instead, the returned documents represent the matching documents in their original state. The find operation did not return the binder or legal pad documents, as their discounted price was greater than 5.