$concat (expression operator)（表达式运算符）

Definition定义

$concat

Concatenates strings and returns the concatenated string.连接字符串并返回连接后的字符串。

$concat has this syntax:

{ $concat: [ <expression1>, <expression2>, ... ] }

The arguments can be any valid expression as long as they resolve to strings. 参数可以是任何有效的表达式，只要它们解析为字符串即可。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。

If the argument resolves to a value of null or refers to a field that is missing, $concat returns null.

Example示例

Create an inventory collection with these documents:使用以下文档创建inventory集合：

db.inventory.insertMany( [
   { _id : 1, item : "ABC1", quarter: "13Q1", description : "product 1" },
   { _id : 2, item : "ABC2", quarter: "13Q4", description : "product 2" },
   { _id : 3, item : "XYZ1", quarter: "14Q2", description : null }
] )

Use the $concat operator to concatenate the item field and the description field with a " - " delimiter:使用$concat运算符用“-”分隔符连接item字段和description字段：

db.inventory.aggregate(
   [
      { $project: { itemDescription: { $concat: [ "$item", " - ", "$description" ] } } }
   ]
)

Output:输出：

{ _id : 1, itemDescription : "ABC1 - product 1" }
{ _id : 2, itemDescription : "ABC2 - product 2" }
{ _id : 3, itemDescription : null }