发布于 2015-09-14 15:17:03 | 282 次阅读 | 评论: 0 | 来源: 网络整理
Use the $cond operator with the following syntax:
{ $cond: [ <boolean-expression>, <true-case>, <false-case> ] }
Takes an array with three expressions, where the first expression evaluates to a Boolean value. If the first expression evaluates to true, $cond returns the value of the second expression. If the first expression evaluates to false, $cond evaluates and returns the third expression.