How do you stop a MongoDB search from being applied recursively to the key-value tree?

Question

Imagine I have this object (written with Ruby literals) stored in a MongoDB:

{"tags" => ["foo", "bar"], 
 "jobs" => [{"title" => "Chief Donkey Wrangler", "tags" => ["donkeys"]}] }

Now, I want to search for objects based on the tags on the first level of data, not the second. I can write a query like this (using the Ruby MongoDB library):

things.find("tags" => {"$exists" => "foo"})

This will obviously match the first example, but it will also match an example like this:

{"tags" => ["baz", "bar"],
 "jobs" => [{"title" => "Trainee Donkey Wrangler", "tags" => ["donkeys", "foo"]}] }

How do I ensure that I am searching only the top-level of keys? I'm interested in knowing the answer in both JavaScript, Ruby and in a language-agnostic way, as I'd like to use MongoDB as a cross-language store.

Obviously, I could pass a map-reduce function to the datastore to pick out the stuff I'm trying to get, but I'm interested to see if it is supported at a higher level (and to reduce the amount of time I spend writing JavaScript map-reduce functions!)

Answer 1

Actually, the query you specify won't match your second example. To match the second example, you'd do:

things.find({"jobs.tags" => "foo"})

There's no recursive application of the query selector.

Answer 2

You're not using $exists properly. $exists does not allow you to search for a match of a field, it just checks for the existence of such a field. I'm guessing that the Ruby MongoDB library is treating your request for 'foo' as equivalent to true, b/c $exists only accepts true/false as an argument

As @kb points out, you want to use the dot notation to reach into the objects.