Model Monetary Data模型货币数据

One-Time Collection Transformation一次性集合转换

A collection can be transformed by iterating over all documents in the collection, converting the monetary value to the decimal type, and writing the document back to the collection.可以通过迭代集合中的所有文档、将货币值转换为decimal类型并将文档写回集合来转换集合。

Scale Factor Transformation:比例因子转换：

Consider the following collection which used the Scale Factor approach and saved the monetary value as a 64-bit integer representing the number of cents:考虑以下集合，该集合使用比例因子方法，并将货币值保存为表示美分数的64位整数：

{ "_id" : 1, "description" : "T-Shirt", "size" : "M", "price" : NumberLong("1999") },
{ "_id" : 2, "description" : "Jeans", "size" : "36", "price" : NumberLong("3999") },
{ "_id" : 3, "description" : "Shorts", "size" : "32", "price" : NumberLong("2999") },
{ "_id" : 4, "description" : "Cool T-Shirt", "size" : "L", "price" : NumberLong("2495") },
{ "_id" : 5, "description" : "Designer Jeans", "size" : "30", "price" : NumberLong("8000") }

The long value can be converted to an appropriately formatted decimal value by multiplying price and NumberDecimal("0.01") using the $multiply operator. 通过使用$multiply运算符将price和NumberDecimal("0.01")相乘，可以将long值转换为格式适当的decimal值。The following aggregation pipeline assigns the converted value to the new priceDec field in the $addFields stage:以下聚合管道将转换后的值分配给$addFields阶段中的新priceDec字段：

db.clothes.aggregate(
  [
    { $match: { price: { $type: "long" }, priceDec: { $exists: 0 } } },
    {
      $addFields: {
        priceDec: {
          $multiply: [ "$price", NumberDecimal( "0.01" ) ]
        }
      }
    }
  ]
).forEach( ( function( doc ) {
  db.clothes.replaceOne( doc );
} ) )

The results of the aggregation pipeline can be verified using the db.clothes.find() query:聚合管道的结果可以使用db.clothes.find()查询进行验证：

{ "_id" : 1, "description" : "T-Shirt", "size" : "M", "price" : NumberLong(1999), "priceDec" : NumberDecimal("19.99") }
{ "_id" : 2, "description" : "Jeans", "size" : "36", "price" : NumberLong(3999), "priceDec" : NumberDecimal("39.99") }
{ "_id" : 3, "description" : "Shorts", "size" : "32", "price" : NumberLong(2999), "priceDec" : NumberDecimal("29.99") }
{ "_id" : 4, "description" : "Cool T-Shirt", "size" : "L", "price" : NumberLong(2495), "priceDec" : NumberDecimal("24.95") }
{ "_id" : 5, "description" : "Designer Jeans", "size" : "30", "price" : NumberLong(8000), "priceDec" : NumberDecimal("80.00") }

If you do not want to add a new field with the decimal value, the original field can be overwritten. 如果不想添加具有decimal值的新字段，则可以覆盖原始字段。The following updateMany() method first checks that price exists and that it is a long, then transforms the long value to decimal and stores it in the price field:下面的updateMany()方法首先检查price是否存在以及它是一个long值，然后将long值转换为decimal并将其存储在price字段中：

db.clothes.updateMany(
  { price: { $type: "long" } },
  { $mul: { price: NumberDecimal( "0.01" ) } }
)

The results can be verified using the db.clothes.find() query:可以使用db.clothes.find()查询来验证结果：

{ "_id" : 1, "description" : "T-Shirt", "size" : "M", "price" : NumberDecimal("19.99") }
{ "_id" : 2, "description" : "Jeans", "size" : "36", "price" : NumberDecimal("39.99") }
{ "_id" : 3, "description" : "Shorts", "size" : "32", "price" : NumberDecimal("29.99") }
{ "_id" : 4, "description" : "Cool T-Shirt", "size" : "L", "price" : NumberDecimal("24.95") }
{ "_id" : 5, "description" : "Designer Jeans", "size" : "30", "price" : NumberDecimal("80.00") }

Non-Numeric Transformation:非数字转换：

Consider the following collection which used the non-numeric model and saved the monetary value as a string with the exact representation of the value:考虑以下集合，该集合使用非数字模型，并将货币值保存为具有该值精确表示形式的string：

{ "_id" : 1, "description" : "T-Shirt", "size" : "M", "price" : "19.99" }
{ "_id" : 2, "description" : "Jeans", "size" : "36", "price" : "39.99" }
{ "_id" : 3, "description" : "Shorts", "size" : "32", "price" : "29.99" }
{ "_id" : 4, "description" : "Cool T-Shirt", "size" : "L", "price" : "24.95" }
{ "_id" : 5, "description" : "Designer Jeans", "size" : "30", "price" : "80.00" }

The following function first checks that price exists and that it is a string, then transforms the string value to a decimal value and stores it in the priceDec field:以下函数首先检查price是否存在以及它是否为string，然后将string值转换为decimal值并将其存储在priceDec字段中：

db.clothes.find( { $and : [ { price: { $exists: true } }, { price: { $type: "string" } } ] } ).forEach( function( doc ) {
  doc.priceDec = NumberDecimal( doc.price );
  db.clothes.replaceOne( doc );
} );

The function does not output anything to the command line. 该函数不会向命令行输出任何内容。The results can be verified using the db.clothes.find() query:可以使用db.clothes.find()查询来验证结果：

{ "_id" : 1, "description" : "T-Shirt", "size" : "M", "price" : "19.99", "priceDec" : NumberDecimal("19.99") }
{ "_id" : 2, "description" : "Jeans", "size" : "36", "price" : "39.99", "priceDec" : NumberDecimal("39.99") }
{ "_id" : 3, "description" : "Shorts", "size" : "32", "price" : "29.99", "priceDec" : NumberDecimal("29.99") }
{ "_id" : 4, "description" : "Cool T-Shirt", "size" : "L", "price" : "24.95", "priceDec" : NumberDecimal("24.95") }
{ "_id" : 5, "description" : "Designer Jeans", "size" : "30", "price" : "80.00", "priceDec" : NumberDecimal("80.00") }

Application Logic Transformation应用程序逻辑转换

It is possible to perform the transformation to the decimal type from within the application logic. 可以在应用程序逻辑中执行到decimal类型的转换。In this scenario the application modified to perform the transformation as it accesses records.在这种情况下，应用程序被修改为在访问记录时执行转换。

The typical application logic is as follows:典型的应用程序逻辑如下：

• Test that the new field exists and that it is of decimal type测试新字段是否存在以及它是否为decimal类型
• If the new decimal field does not exist:如果新的decimal字段不存在：
  • Create it by properly converting old field values通过正确转换旧字段值创建它
  • Remove the old field删除旧字段
  • Persist the transformed record保留转换后的记录