New in version 3.6.

Compare Two Fields from A Single Document

Consider an monthlyBudget collection with the following documents:

{ "_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" ] } } )

The operation returns the following results:

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

Using $expr With Conditional Statements

Some queries require the ability to execute conditional logic when defining a query filter. The aggregation framework provides the cond operator for applying conditional logic to a pipeline stage. You can use $expr with the $cond operator to query based on conditional modifications to the source data.

Consider a supplies collection with the following documents. Each document in the supplies collection represents an item, its quantity, and its price:

{ "_id" : 1, "item" : "binder", "qty" : 100 , "price" : 12 }
{ "_id" : 2, "item" : "notebook", "qty" : 200 , "price" : 8 }
{ "_id" : 3, "item" : "pencil", "qty" : 50 , "price" : 6 }
{ "_id" : 4, "item" : "eraser", "qty" : 150 , "price" : 3 }
{ "_id" : 5, "item" : "legal pad", "qty" : 42 , "price" : 10}

An application automatically applies a 50% discount to the price of any item with a quantity of more than 100. Otherwise, the application applies a 25% discount to the price.

A user needs to know which items in the supplies collection have a discounted price of less than 5. The user writes a query that:

1. Uses conditional logic to identify the discount applicable to a given document.
2. Calculates that document’s discounted price.
3. Only returns documents whose discounted price is less than 5.

The query returns only documents from the supplies collection whose discounted price is less than 5.

The following operation implements the previous steps using the $expr operator to access the cond and $lt aggregation operators:

db.supplies.find( {
    $expr: {
       $lt:[ {
          $cond: {
             if: { $gte: ["$qty", 100] },
             then: { $multiply: ["$price", 0.50] },
             else: { $multiply: ["$price", 0.75] }
           }
       },
       5 ] }
} )

• $cond modifies the input document depending on the value of qty. Specifically, it compares the qty using $gte and modifies the price using $multiply.
  • The document _id : 1 has a qty of 100. $cond returns the result of multiplying the price by 0.50, effectively applying a 50% discount, yielding a value of 6.
  • The document id : 3 has a qty of 50. $cond returns the result of multiplying the price by 0.75, effectively applying a 25% discount, yielding a value of 4.
• $lt compares the returned value to the provided value 5.
  • The document _id : 1 returns 6 from cond. Since $lt [6, 5] evaluates to false, the expression does not return the document.
  • The document _id : 3 returns 4 from cond. Since $lt [4, 5] evaluates to true, the expression returns the document.

find() continues to apply $cond to each document in the supplies collection. The operation returns the following results:

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

$cond modifies the input documents as part of the query filtering but does not persist the modifications to disk. The results represent the matching documents in their original state. The find operation did not return the binder or legal pad documents, as their modified price was greater than 5. Specifically:

• binder has a quantity of 200. The $cond logic applies a .75 multiplier which resolves to 6. Since 6 is greater than 5, find() filters out the binder document.
• legal pad has a quantity of 42. The $cond logic applies a .50 multiplier which resolves to 5. Since the operation used the $lt operator, the modified price is not less than 5 and find() filters out the legal pad document.