Mongo的查找操作未按预期工作

Question

Below is the code that works well:

for(i=0;i<distinct_branch.length;i++){
                    myobj = {}
                    myobj['branch'] = distinct_branch[i]
                    myobj['total_re'] = 0
                    myobj['present'] = 0
                    myobj['absent'] = 0
                    myobj['weekly_off'] = 0
                    myobj['leave'] = 0
                    myobj['total_marked'] = 0
                    myobj['pending_attend'] = 0
                    myobj['dat_compliance'] = 0
                    myobj['pending_dat_compliance'] = 0
                    for(j=0;j<results.length;j++){
                        if(results[j].counter){
                            if(results[j].counter.state){
                                if(distinct_branch[i]==results[j].counter.state){
                                    myobj['total_re'] = myobj['total_re'] + 1
                                    if(results[j].attendance == 'P') {
                                        myobj['present'] = myobj['present'] + 1
                                        myobj['total_marked'] = myobj['total_marked'] + 1
                                    }
                                    else{
                                        if(results[j].attendance == 'A') {
                                            myobj['absent'] = myobj['absent'] + 1
                                            myobj['total_marked'] = myobj['total_marked'] + 1
                                        }
                                        else{
                                            if(results[j].attendance == 'O') {
                                                myobj['weekly_off'] = myobj['weekly_off'] + 1
                                                myobj['total_marked'] = myobj['total_marked'] + 1
                                            }
                                            else{
                                                if(results[j].attendance == 'L') {
                                                    myobj['leave'] = myobj['leave'] + 1
                                                    myobj['total_marked'] = myobj['total_marked'] + 1
                                                }
                                                else{
                                                    if(results[j].attendance == 'Pending') {
                                                        myobj['pending_attend'] = myobj['pending_attend'] + 1
                                                    }
                                                }
                                            }
                                        }
                                    }
                                    myobj['dat_compliance'] = ( myobj['total_marked'] / myobj['total_re'])*100
                                    myobj['pending_dat_compliance'] = 100 - myobj['dat_compliance']
                                }
                            }
                        }
                    }
                    returndata.push(myobj)
                }

Now I want that "returndata should have one more property which is the result of a find query" Below is the query:

imports.db.mdb.collection('sales_details').find({'user.empcode':'481625'},function(err,sale_data){
myobj['test']=sale_data;
});
returndata.push(myobj);

//But I have tried each and everything and I am unable to perform this.Node is performing the operation of pushing first and then the query is executed(find). I even pushed myobj in the callback of find by comparing if I is equal to distinct_branch.length but that too is not working. Thanks in advance :)

Answer 1

The find method work exactly as expected. ;-)

nodejs (googles V8 VM) ist designed to be reactive. This is due to the fact that there's only one thread running in a nodejs process. As a consequence long running IO-operations perform in its own process parallel to the programm that triggers the IO-access.

That means if you perform a db.collection.find({}, callback); the vm requests the mongodb process to search for the data and than returns the control back to the calling programm/function which executes the next statement then the next one and so on. Because a DB search lasts many longer than a value assignment to a variable, the added value is undefined.

As soon as the DB returns the search result, it executes the spezified callback and passing an error variable and a result variable to it. You can use the callback to synchronize the find method with the caller. ie execute the send method inside of the callback, when it is sure that the data is available.

An example: Let me assume that you use the find method to answer a (let's say) express route request.

...
router.get(url, function(req,res,next)
{
   db.find({...}, function(err, sale_data)
   {
       if ( err )
       {
           next(err);
       }
       myobj['test']=sale_data;    
       returndata.push(myobj);
       resp.json(returndata);
   });
});

Then the request is send as soon as the data is available.

Synchronizing is easy for your example, but as soon as you use the returned data to start a new asynchronous request to the DB and use the return data to start ... Then it is best practise to us a synchronization framework like async or promises like handled by Kris Kowalskis Q .