For details on specific operator, including syntax and examples, click on the specific operator to go to its reference page.有关特定运算符的详细信息，包括语法和示例，请单击特定运算符以转到其参考页。
Sets the value of a field if an update results in an insert of a document. 如果更新导致插入文档，则设置字段的值。Has no effect on update operations that modify existing documents.对修改现有文档的更新操作没有影响。