ruby_mongo_helpers.rb

class MongoHelpers
  # Sum over fields
  def self.sum(collection, fields, group_by, conditions=nil)
    map = <<-EOD
      function() {
        self = this;

        key = {};
        if(group_by && group_by.length > 0) {
          group_by.forEach(function (g) {
            key[g] = self[g];
          });
        } else {
          key["group"] = true;
        }

        vals = {};
        fields.forEach(function (f) {
          vals[f] = self[f];
        })

        emit(key, vals)
      }
    EOD

    reduce = <<-EOD
      function (key, values) {
        totals = {};
        values.forEach(function (v) {
          fields.forEach(function (f) {
            if(v[f]) {
              if(totals[f]) {
                if(typeof(v[f]) == "object" && typeof(v[f].concat) == "function") {
                  totals[f] = totals[f].concat(v[f]);
                } else {
                  totals[f] += v[f];
                }
              } else {
                totals[f] = v[f];
              }
            }
          });
        });
        return totals;
      }
    EOD

    MongoUtil.connection[collection].map_reduce(map, reduce, {
      :query => conditions,
      :scope => {
        "group_by" => group_by,
        "fields" => fields
      },
      :out => 'inline'
    }).find
  end

  # Helper function for renaming a field in all documents in a collection
  def self.rename_field(collection, old_name, new_name)
    func = <<-EOD
      function rename_field(collection, old_name, new_name) {
        criteria = {}
        criteria[old_name] = { $exists: true }

        old_field = {}
        old_field[old_name] = true

        db[collection].find(criteria, old_field).forEach(function (doc) {
          new_field = {}
          new_field[new_name] = doc[old_name]

          db[collection].update({_id: ObjectId(doc._id)}, {
            $set: new_field,
            $unset: old_field
          });
        });
      }
    EOD

    MongoUtil.connection.eval(func, collection, old_name, new_name)
  end
end