发布于 2015-09-14 15:19:37 | 166 次阅读 | 评论: 0 | 来源: 网络整理
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$ifNull
Use the $ifNull operator with the following syntax:
{ $ifNull: [ <expression>, <replacement-if-null> ] }
Takes an array with two expressions. $ifNull returns
the first expression if it evaluates to a non-null
value. Otherwise, $ifNull returns the second
expression’s value.