MongoDB聚合：$ exists的任何值

Question

I'm trying to leverage MongoDB aggregation for a RESTful api, but got stuck with the following case. Suppose, I have Subscriptions model, which looks like this:

var mongoose = require('mongoose'),
  Schema = mongoose.Schema;

var SubscriptionSchema = new Schema({
  // ...
  cancelled: Date
});

This cancelled property can be either undefined , if the subscription is active, or become a Date of the user's cancel action.

Now, I have a route GET /me/subscriptions , which aggregates subscriptions and has an optional query parameter: cancelled=true (show only cancelled) or cancelled=false (show only active). If not specified, should return any subscription (active or cancelled).

var express = require('express'),
  router = express.Router(),
  Subscription = require('../../models/subscription');

router.get('/me/subscriptions', function(req, res, next) {
  var cancelled = req.query.cancelled === 'true' ? { $exists: true } :
    req.query.cancelled === 'false' ? { $exists: false } :
    { $exists: { $or: [ true, false ] } }; // wrong logic here

  return Subscription.aggregate([
      { $match: { user: req.user._id, cancelled: cancelled }},
      { $project: {
        // ...
      }}
    ])
    .exec()
    // ...
});

module.exports = router;

It works perfectly if I pass the above mentioned query parameter, but fails to find models if no parameter is specified (or if it is not equal to either true or false ). I have tried lots of stuff instead of the wrong line (in the $match pipeline):

cancelled: {}
cancelled: void 0
cancelled: { $exists: { $or: [ true, false ] } }
cancelled: { $exists: { $in: [ true, false ] } }
cancelled: { $exists: [ true, false ] } // obviously wrong, but hey
cancelled: null // obviously wrong, too
cancelled: { $or: [ { $exists: false }, { $exists: true } ] } // can't use $or here, but still, hey

The only solution is see now is something like this, comparing against a value which is not undefined and not of type Date , but it seems way too hacky.

cancelled: { $ne: 'some-impossible-value' }

Any help is very much appreciated.

Answer 1

I think a little tweaking will satisfy the condition as you want.

var express = require('express'),
  router = express.Router(),
  Subscription = require('../../models/subscription');

router.get('/me/subscriptions', function(req, res, next) {
  var match_query = {user: req.user._id};

  if (req.query.cancelled === 'true') {
      match_query.cancelled = {$exists:true};
  } else if(req.query.cancelled === 'false') {
      match_query.cancelled = {$exists:false};
  }

  return Subscription.aggregate([
      { $match: match_query},
      { $project: {
        // ...
      }}
    ])
    .exec()
    // ...
});

module.exports = router;

You don't need to add { $exists: { $or: [ true, false ] } , just don't add anything to query if you don't get true or false.

I haven't checked code for syntax error, but logically it will work.

Answer 2

I would restructure the $match pipeline as follows (requires installing the momentjs library):

router.get('/me/subscriptions', function(req, res, next) {
    var match = {
        "$match": {
            "user": req.user._id,
            "cancelled": {}
        }
    };

    if (req.query.cancelled === 'true' || req.query.cancelled === 'false') {
        match["$match"]["cancelled"]["$exists"] = JSON.parse(req.query.cancelled);
    } else if(moment(req.query.cancelled, moment.ISO_8601, true).isValid()){
        match["$match"]["cancelled"] = moment(req.query.cancelled).toDate();
    }
    else {
        match["$match"]["cancelled"]["$exists"] = false;
    }       

    return Subscription.aggregate([match,
        { "$project": {
          // ...
        }}
    ]).exec()
    // ...
});