用猫鼬增加数字字段？ 用于在.find上对结果进行排序

Question

I have a mongoose model that looks like this:

module.exports = mongoose.model('Item', {
    text : String,
    position: Number
});

And I'm looking to have a Position field that increments on something like the .length of all the documents, for sorting the results of a .find of All:

// get All Items
app.get('/itemsList', function(req, res) {

    // use mongoose to get all items in the database
    Item.find({
        sort : { position: 1 } // sort by Ascending Position
    }. function(err, items) {

        // if there is an error retrieving, send the error. nothing after res.send(err) will execute
        if (err)
            res.send(err)

        res.json(items); // return all items in JSON format
    });
});

Is there a way to auto-fill a number for the Position field with some javascript in node.js?

// create an item
app.post('/api/item', function(req, res) {

    // create an item, information comes from AJAX request from Angular
    Item.create({
        text : req.body.text,
        position:
        // something using ++items.length
    }, function(err, item) {
        if (err)
            res.send(err);
    });

});

Answer 1

Mongoose lets you hook into the save , validate and remove methods and execute code before and after they're executed.

This code can be asynchronous. For example, in your case you could probably do this:

var schema = mongoose.Schema({
    text : String,
    position: Number
});
schema.pre("validate", function(next) {
    var doc = this;

    // If 'position' is not filled in, fill it in.
    // Not using !position because 0 might be a valid value.
    if(typeof position !== "number") {
        // Count the number of Items *
        mongoose.model("Item").count(function(err, num) {
            // If there was an error, pass it to next().
            if(err)
                return next(err);

            // Update the position, then call next();
            doc.position = num;
            return next();
        });
    } else {
        //  There is no need to count, so call next().
        next();
    }
});
module.exports = mongoose.model('Item', schema);

More here .

Before validation starts, the number of Items is counted. Afterwards, the position is set.
Validation and other pre-validator ** hooks will not commence until the above code is ready.

* I'm using mongoose.model here to fetch the model because the model is not compiled yet (that happens a bit below).

** The documentation shows you how you can make multiple pre-validator hooks execute in parallel. I've chosen not to do this in this example because the code is easier to read and because you might actually need the validators to run sequentially.

---

In the pre-validation hook, you could place some logic in the else-case. When inserting an Item with an existing position value, you'll want to move every record down. You can do this by doing the following:

1. Use this.isModified("position") to check if the value was changed since you last saved. You might also need doc.isNew().
2. Check if there is an existing document with the same position . Something like Item.where({_id: {$ne: this._id}, position: this.position}).count()
3. If there is, execute: Item.update({position: {$gte: this.position}}, {position: {$inc: 1}}, {multi: 1})
4. Then call next() to save your doc.

The above should work. It will leave gaps when you remove documents however.

Also, look into indexes. You'll want to add one on the position field. Perhaps even a unique index .

Answer 2

Per @RikkusRukkus's steps for moving records down, here's logic for the else-case (to be tested)

// load mongoose since we need it to define a schema and model
var mongoose = require('mongoose');

var ItemSchema = mongoose.Schema({
    text : String,
    position: Number
});

// before validation starts, the number of Items is counted..afterwards, the position is set
ItemSchema.pre("validate", function(next) {

    var doc = this;

    // if 'position' is not filled in, fill it in..not using !position because 0 might be a valid value
    if(typeof position !== "number") {
        // count the number of Items *
        // use mongoose.model to fetch the model because the model is not compiled yet
        mongoose.model("Item").count(function(err, num) {
            // if there was an error, pass it to next()
            if(err)
                return next(err);

            // set the position, then call next();
            doc.position = num;
            return next();
        });
    } else if(this.isModified("position") || this.isNew()) {
        // check if there is an existing document with the same position
        // use mongoose.model to fetch the model because the model is not compiled yet
        mongoose.model("Item").where({_id: {$ne: this._id}, position: this.position}).count( function (err, count) {

            // if there was an error, pass it to next()
            if(err)
                return next(err);

            // if there is a doc with the same position, execute an update to move down all the $gte docs
            if(count > 0) {
                // use mongoose.model to fetch the model because the model is not compiled yet
                mongoose.model("Item").update({position: {$gte: this.position}}, {position: {$inc: 1}}, {multi: 1}, function(err, numAffected) {
                    // Call next() (with or without an error)
                    next(err);
                });

            } else {
                //  there are no docs that need to move down, so call next()
                next();
            }
        });
    } else {
        //  there is no need to count or update positions, so call next()
        next();
    }
});

module.exports = mongoose.model('Item', ItemSchema);