$expr

Compare Two Fields from A Single Document比较单个文档中的两个字段

Consider an monthlyBudget collection with the following documents:考虑一个包含以下文档的monthlyBudget集合：

{ "_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 $expr to find documents where the spent amount exceeds the budget:以下操作使用$expr查找spent（支出）金额超过budget（预算）的文档：

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

The operation returns the following results:该操作返回以下结果：

{ "_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 $expr With Conditional Statements将$expr与条件语句一起使用

Some queries require the ability to execute conditional logic when defining a query filter. 在定义查询筛选器时，某些查询需要执行条件逻辑的能力。The aggregation pipeline provides the $cond operator to express conditional statements. 聚合管道提供$cond运算符来表达条件语句。By using $expr with the $cond operator, you can specify a conditional filter for your query statement.通过将$expr与$cond运算符一起使用，可以为查询语句指定条件筛选器。

Create a sample supplies collection with the following documents:使用以下文档创建样例supplies集合：

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:假设下个月即将进行的销售，您希望对价格进行折扣，以便：

• If qty is greater than or equal to 100, the discounted price will be 0.5 of the price.如果qty大于或等于100，折扣价格将为price的0.5。
• If qty is less than 100, the discounted price is 0.75 of the price.如果qty少于100，折扣价格为price的0.75。

Before applying the discounts, you would like to know which items in the supplies collection have a discounted price of less than 5.在应用折扣之前，您想知道supplies集合中哪些商品的折扣价格低于5。

The following example uses $expr with $cond to calculate the discounted price based on the qty and $lt to return documents whose calculated discount price is less than NumberDecimal("5"):以下示例使用$expr和$cond来计算基于qty的折扣价格，并使用$lt来返回计算出的折扣价格小于NumberDecimal("5")的文档：

// Aggregation expression to calculate discounted price用于计算折扣价格的聚合表达式

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使用聚合表达式查询耗材集合

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).下表显示了每张单据的折扣价，以及折扣价是否小于NumberDecimal("5")（即单据是否满足查询条件）。

The db.collection.find() operation returns the documents whose calculated discount price is less than NumberDecimal("5"):操作返回计算出的折扣价格小于NumberDecimal("5")的文档：db.collection.find()操作返回计算出的折扣价格小于NumberDecimal("5")的文档：

{ "_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 $cond calculates an effective discounted price, that price is not reflected in the returned documents. Instead, the returned documents represent the matching documents in their original state. 即使$cond计算了一个有效的折扣价格，该价格也不会反映在返回的文档中。相反，返回的文档表示原始状态下的匹配文档。The find operation did not return the binder or legal pad documents, as their discounted price was greater than 5.查找操作没有返回binder或legal pad文档，因为它们的折扣价大于5。