$unset¶

Definition定义¶

$unset¶

The $unset operator deletes a particular field. Consider the following syntax:$unset运算符删除特定字段。考虑下面的语法：

{ $unset: { <field1>: "", ... } }

The specified value in the $unset expression (i.e. "") does not impact the operation.$unset表达式中的指定值（即""）不会影响操作。

To specify a <field> in an embedded document or in an array, use dot notation.若要在嵌入式文档或数组中指定一个<field>，请使用点表示法。

Behavior行为¶

If the field does not exist, then $unset does nothing (i.e. no operation).如果该字段不存在，则$unset不执行任何操作（即无操作）。

When used with $ to match an array element, $unset replaces the matching element with null rather than removing the matching element from the array. 与$一起使用以匹配数组元素时，$unset将匹配元素替换为null，而不是从数组中删除匹配元素。This behavior keeps consistent the array size and element positions.此行为使数组大小和元素位置保持一致。

Example示例¶

The following update() operation uses the $unset operator to remove the fields quantity and instock from the first document in the products collection where the field sku has a value of unknown.下面的update()操作使用$unset运算符从products集合中的第一个文档中删除字段quantity和instock，其中字段sku的值为unknown。

db.products.update(
   { sku: "unknown" },
   { $unset: { quantity: "", instock: "" } }
)