Mongodb：collection.find（）中的动态查询对象

Question

I am working on a Node.js + MongoDB application. The application inserts some records in the MongoDB. For example lets take below simple record:

{
  "name": "Sachin",
  "age" : 11,
  "class": 5,
  "percentage": 78,
  "rating": 5
}

Now end user can set different rule for which they want to get the notification/alert when a specific condition is satisfied. For example we can have a rule like:

1) Rule1: Generate notification/alert if "percentage" is less than 40

In order to achieve this, I am using Replication and tailable cursor. So whenever a new record gets added in the collection I get an record in the tailable cursor.

coll = db.collection('oplog.rs');

options = {
    tailable: true,
    awaitdata: true,
    numberOfRetries: -1
};

var qcond = {'o.data.percentage':{$gt:40}};

coll.find(qcond, options, function(err, cur) {

    cur.each(function(err, doc) {

        //Perform some operations on received document like 
        //adding it to other collection or generating alert

    }); //cur.each

}); //find

Everything works fine till this point.

Now problem starts when enduser wants to add another rule at runtime say:

2) Rule2: Generate notification/alert if "rating" is greater than 8

Now I would like to consider this condition/rule as well when querying the tailable cursor. But the current cursor is already in a waiting state based on the conditions given as per Rule1 only.

Is there any way to update the query conditions dynamically so that I can include conditions for Rule2 as well?

I tried searching but couldn't find a way to achieve this.

Does anyone have any suggestion/pointers to tackle this situation?

Answer 1

No. You can't modify a cursor once it's open on the server. You'll need to terminate the cursor and reopen it to cover both conditions, or open a second cursor to cover the second condition.