MongoDB — Setting a conditional statement in .find()

问题

for aggregate, $cond can be used to form an if-statement. I'm wonder if there's a similar function for .find()

When I tried using $cond in find(), I get

"unknown top level operator: $cond"

I'm trying to do the following (in pseudo code):

if condition is True:
    set filter for find to thing_1
else:
    set filter for find to thing_2

回答1:

If you are on Mongo 3.6 or greater, you could try using the $expr operator. Which would then allow you to use $cond.

$expr Allows the use of aggregation expressions within the query language.

Their example can be found here

db.supplies.find( {
    $expr: {
       $lt:[ {
          $cond: {
             if: { $gte: ["$qty", 100] },
             then: { $divide: ["$price", 2] },
             else: { $divide: ["$price", 4] }
           }
       },
       5 ] }
} )

来源：https://stackoverflow.com/questions/43082423/mongodb-setting-a-conditional-statement-in-find