MongoDB eval() with arguments

Question

I'm getting a strange result when trying to use eval with the args argument. The following works fine:

> db.eval(function(coll) {
  var res = db[coll].find({});
  return(res.count());
}, ['KenColl'])
1438

But when I pass a second argument, I always get empty results, even if I don't use it:

> db.eval(function(coll, query) {
  var res = db[coll].find({});
  return(res.count());
}, ['KenColl', {}])
0

Am I misunderstanding something about eval and args? I'm running version 2.4.3 of both mongod and the MongoDB shell.

jimoleary · Accepted Answer · 2013-06-05 16:20:48Z

1

For db.eval you shouldn't pass the arguments as an array, just pass them into the function.

The following example should work:

db.eval(function(coll, query) {
  var res = db[coll].find(query);
  return(res.count());
}, 'KenColl', {})

p.s. your first example only works because in javascript db['KenColl'] === db[['KenColl']]

answered Jun 5, 2013 at 16:20

jimoleary

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5 Comments

Ken Williams Over a year ago

Did this change in recent versions of the DB? I'm looking at the "Definitive Guide" book by Chodorow and Dirolf, and on page 105 it shows db.eval taking an array as the second argument. Or is that just an error in the text?

Asya Kamsky Over a year ago

if you pass it an array and the function expects a single argument then the argument will be an array with however many arguments you put in it.

Ken Williams Over a year ago

@AsyaKamsky: in the book though, it shows db.eval("function(x,y,z){ return x+y+z; }", [num1, num2, num3]), which I just confirmed in my shell doesn't work as expected.

Asya Kamsky Over a year ago

yeah, I just checked - that syntax doesn't work with spidermonkey shell with 2.0 nor with V8 shell in 2.4. That book was written circa v1.4(?) unless you have a new printing...

Asya Kamsky Over a year ago

I just confirmed, this example is NOT in the new printing of the book so you are looking at the older one. New one: amazon.com/MongoDB-Definitive-Guide-Kristina-Chodorow/dp/…

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