MongoDB $mod操作符

$mod操作符是什么

MongoDB提供了各种算术表达式操作符，$mod操作符就是其中之一。$mod操作符用于聚合管道阶段中，用于将一个数除以另一个数并返回余数。换句话说，第一个数是被除数，第二个数是除数。

$mod操作符的语法

{ $mod: [ < expression 1 >, < expression 2 > ] }

可以是解析为数字的任何有效表达式。

示例

在以下示例中，我们正在处理：

Database: javaTpoint
Collection: items
Document: Ten documents that contain the details of the items

>db.items.find().pretty()
{
    {
        "_id" : 1,
        "item_name" : "Apple",
        "total_Price" : null,
        "quantity" : 40,
    }
    {
        "_id" : 2,
        "item_name" : "Banana",
        "total_Price" : 1000,
        "quantity" : 72,
    }
    {
        "_id" : 3,
        "item_name" : "Cherry",
        "total_Price" : 215,
        "quantity" : 25,
    }
    {
        "_id" : 4,
        "item_name" : "Apple",
        "total_Price" : null,
        "quantity" : 25,
    }
    {
        "_id" : 5,
        "item_name" : "Banana",
        "total_Price" : 400,
        "quantity" : 35,
    }
    {
        "_id" : 6,
        "item_name" : "Banana",
        "total_Price" : 510,
        "quantity" : 100,
    }
    {
        "_id" : 7,
        "item_name" : "Cherry",
        "total_Price" : 500,
        "quantity" : 41,
    }
    {
        "_id" : 8,
        "item_name" : "Rasbhari",
        "total_Price" : 80,
        "quantity" : "Ten",
    }
    {
        "_id" : 9,
        "item_name" : "Banana",
        "total_Price" : 205,
        "quantity" : 10,
    }
    {
        "_id" : 10,
        "item_name" : "Apple",
        "total_Price" : 95,
        "quantity" : null,
    }
}

示例1：使用$mod运算符找到余数

在这个示例中，我们将使用$mod运算符将”total_Price”字段的值除以”quantity”字段的值，并找到余数。

db.item.aggregate(
 [
   {match: {item_name : "Banana"}},
      {project: 
              {
                item_name : 1,
                total_Price : 1,
                quantity : 1,
                remainderValue : {mod: ["total_Price", "$quantity"]}
               }
      }
 ]
)

输出：

{
    "_id" : 2,
    "item_name" : "Banana",
    "total_Price" : 1000,
    "quantity" : 72,
    "remainderValue" : 64
}
{
    "_id" : 5,
    "item_name" : "Banana",
    "total_Price" : 400,
    "quantity" : 35,
    "remainderValue" : 15
}
{
    "_id" : 6,
    "item_name" : "Banana",
    "total_Price" : 510,
    "quantity" : 100,
    "remainderValue" : 10
}
{
    "_id" : 9,
    "item_name" : "Banana",
    "total_Price" : 205,
    "quantity" : 10,
    "remainderValue" : 5
}  

示例2：数据类型错误

$mod运算符仅支持数值数据类型。如果值是字符串，$mod运算符会返回错误。

{ "_id" : 8, "item_name" : "Rasbhari", "total_Price" : 80, "quantity" : "Ten", }

db.item.aggregate(
 [
   {match: {item_name : "Rasbhari"}},
      {project: 
              {
                item_name : 1,
                total_Price : 1,
                quantity : 1,
                remainderValue : {mod: ["total_Price", "$quantity"]}
               }
      }
 ]
)

输出:

uncaught exception: Error: command failed: {
    "ok" : 0,
    "errmsg" : "$mod only supports numeric types, not string and double",
    "code" : 16611,
    "codeName" : "Location16611"
} : aggregate failed :
_getErrorWithCode@src/mongo/shell/utils.js:25:13
doassert@src/mongo/shell/assert.js:18:14
_assertCommandWorked@src/mongo/shell/assert.js:639:17
assert.commandWorked@src/mongo/shell/assert.js:729:16
DB.prototype._runAggregate@src/mongo/shell/db.js:266:5
DBCollection.prototype.aggregate@src/mongo/shell/collection.js:1058:12
@(shell):1:1   

示例3：空值

在这个示例中，我们将使用$mod运算符在apple中通过将”total_Price”字段的值除以”quantity”字段来找到余数。

db.item.aggregate(
 [
   {match: {item_name : "Apple"}},
      {project: 
              {
                item_name : 1,
                total_Price : 1,
                quantity : 1,
                remainderValue : {mod: ["total_Price", "$quantity"]}
               }
      }
 ]
)

输出：

{
    "_id" : 1,
    "item_name" : "Apple",
    "total_Price" : null,
    "quantity" : 40,
    "remainderValue" : null
}
{
    "_id" : 4,
    "item_name" : "Apple",
    "total_Price" : null,
    "quantity" : 25,
    "remainderValue" : null    
}
{
    "_id" : 10,
    "item_name" : "Apple",
    "total_Price" : 95,
    "quantity" : null,
    "remainderValue" : null
}