How to build a conditional query in Mongoose?

Question

The following code works with no querystrings or one querystring only. In other words, simply going to /characters returns all characters. But if you were to specify a querystring parameter /characters?gender=male it will return only male characters.

How could I extend this to work with either 1, 2, 3, or no querystrings? I would really prefer to avoid writing 8 or 9 different if-statements for each case. I was hoping Mongoose would simply ignore a $where clause if it's null or undefined, but that is not the case (see commented out code).

  var gender = req.query.gender;
  var race = req.query.race;
  var bloodline = req.query.bloodline;

  var query = Character.find();

  if (gender)
    query = query.where('gender').equals(gender);
  if (race)
    query = query.where('race').equals(race);
  if (bloodline)
    query = query.where('bloodline').equals(bloodline);

  /*
  query
    .where('gender').equals(new RegExp('^' + gender + '$', 'i'))
    .where('race').equals(new RegExp('^' + race + '$', 'i'))
    .where('bloodline').equals(new RegExp('^' + bloodline + '$', 'i'));
  */

  query.exec(function(err, characters) {
    if (err) throw err;
    res.send(characters);
  });

Edit: Well I guess I could do it with 7 if-statements for now. Unless someone finds a more elegant solution.

Edit 2:

Thanks guys. It's difficult to pick one answer because both of you have helped me to achieve this concise solution. Here is the entire thing now.

var conditions = {};

for (var key in req.query) {
  if (req.query.hasOwnProperty(key)) {
    conditions[key] = new RegExp('^' + req.query[key] + '$', 'i');
  }
}

var query = Character.find(conditions);
query.exec(function(err, characters) {
  if (err) throw err;
  res.send({ characters: characters });
});

Answer 1

You don't need to call Query#where repeatedly, since you can pass all the conditions to Mongoose Model#find as:

var filteredQuery = {},
  acceptableFields = ['gender', 'race', /* etc */ ];

acceptableFields.forEach(function(field) {
  req.query[field] && filteredQuery[field] = req.query[field];
});

var query = Character.find(filteredQuery);

You'll also want to sanitize req.query depending on the allowed parameters you have in mind.