MongoDB: Updating documents using data from the same document

Question

I have a list of documents, each with lat and lon properties (among others).

{ 'lat': 1, 'lon': 2, someotherdata [...] } 
{ 'lat': 4, 'lon': 1, someotherdata [...] }
[...]

I want to modify it so that it looks like this:

{ 'coords': {'lat': 1, 'lon': 2}, someotherdata [...]} 
{ 'coords': {'lat': 4, 'lon': 1}, someotherdata [...]}
[...]

So far I've got this:

db.events.update({}, {$set : {'coords': {'lat': db.events.lat, 'lon': db.events.lon}}}, false, true)

But it treats the db.events.lat and db.events.lon as strings. How can I reference the document's properties?

Cheers.

Answer 1

From the CLI? I think you have to pull the values out first and assign the value into a variable. Then run your update command.

Or (I haven't tried) remove 'db' from the string. events.lat and events.lon If it works, you will still have multiple values, the old values for "lat" and "lon" and the new array you created.

Answer 2

According to this comment and its reply, you cannot reference the current document's properties from within an update().

You'll have to iterate through all the documents and update them like this:

db.events.find().forEach(
  function (e) {
    // update document, using its own properties
    e.coords = { lat: e.lat, lon: e.lon };

    // remove old properties
    delete e.lat;
    delete e.lon;

    // save the updated document
    db.events.save(e);
  }
)

Such a function can also be used in a map-reduce job or a server-side db.eval() job, depending on your needs.