Seamus Abshere Seamus Abshere - 3 months ago 14
Ruby Question

How can I stringify a BSON object inside of a MongoDB map function?

I have documents with field

xyz
containing

{ term: "puppies", page: { skip: 1, per_page: 20 } } // not useful as a composite key...
{ page: { skip: 1, per_page: 20 }, term: "puppies" } // different order, same contents


For the sake of determining the "top" values in xyz, I want to map them all to something like

emit('term="puppies",page={ skip: 1, per_page: 20 }', 1); // composite key


but I can't get the embedded objects into a meaningful strings:

emit('term="puppies",page=[object bson_object]', 1); // not useful


Any suggestions for a function to use instead of
toString()
?

# return the top <num> values of <field> based on a query <selector>
#
# example: top(10, :xyz, {}, {})
def top(num, field, selector, opts = {})
m = ::BSON::Code.new <<-EOS
function() {

var keys = [];

for (var key in this.#{field}) {
keys.push(key);
}

keys.sort ();

var sortedKeyValuePairs = [];

for (i in keys) {
var key = keys[i];
var value = this.#{field}[key];

if (value.constructor.name == 'String') {
var stringifiedValue = value;
} else if (value.constructor.name == 'bson_object') {
// this just says "[object bson_object]" which is not useful
var stringifiedValue = value.toString();
} else {
var stringifiedValue = value.toString();
}

sortedKeyValuePairs.push([key, stringifiedValue].join('='));
}

// hopefully we'll end up with something like
// emit("term=puppies,page={skip:1, per_page:20}")
// instead of
// emit("term=puppies,page=[object bson_object]")
emit(sortedKeyValuePairs.join(','), 1);
}
EOS
r = ::BSON::Code.new <<-EOS
function(k, vals) {
var sum=0;
for (var i in vals) sum += vals[i];
return sum;
}
EOS
docs = []
collection.map_reduce(m, r, opts.merge(:query => selector)).find({}, :limit => num, :sort => [['value', ::Mongo::DESCENDING]]).each do |doc|
docs.push doc
end
docs
end

Answer

Given that MongoDB uses SpiderMonkey as its internal JS engine, can't you use JSON.stringify (will work even if/when MongoDB switches to V8) or SpiderMonkey's non-standard toSource method?

(sorry, can't try it ATM to confirm it'd work)

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