Mongodb aggregation framework query

[juhi bhatia]

I can do a mongodb aggregation query like:

db.elements.aggregate({$project : {"weekId" : 1, "monthId" : 1}}, {$match : {"weekId" : {$gt : 3*4}}})

the above query gives correct results.

I am trying to fire the query below:

db.elements.aggregate({$project : {"weekId" : 1, "monthId" : 1}}, {$match : {"weekId" : {$gt : "monthId" * 4}}})

the query evaluates w/o any errors, but does not give correct answer. I also tried "$monthId" instead of "monthId", I get the same results but incorrect.

My question is, can i do something like that in mongodb aggregation? If yes, what is the correct query mechanism?

[Ross Lawley]

Hi,

I don't think you can dynamically match in that way, but you dynamically test when doing your projection and match on those results.  So combining the $cond and  $multiply operators you can project a new field to match against.  See below:

db.elements.aggregate(

  {$project : {

    "weekId" : 1, 

    "monthId" : 1, 

    "test": { 

      $cond: [{$gt : ["$weekId", {$multiply: ["$monthId", 4]}]}, 1, 0 ]}

  }}, 

  {$match : { "test" : 1}},

  {$project : {

    "weekId" : 1, 

    "monthId" : 1,

  }}

)

Ross