How can I create a mongoose virtual field that lists the length of a subdocument?

Question

I have a node.js REST api route which returns projects objects as json minus the tasks:

   exports.list = function(req, res) {
    return Project.find(null,'-tasks',null,function(err, projects) {
     if (!err) {
      return res.json(projects);
     } else {
      return res.send(err);
     }
    });
   };

This is intended to be used for navigation elements that list all projects but won't be needing the individual tasks (which number in the 100s). This works well but now I need to include a count of the number of tasks in the navigation so I added this to my model:

var ProjectSchema = Schema({
  name    : String,
  tasks : [{ 
    name : String,
    state   : String }]
},
{
    toObject: { virtuals: true },
    toJSON: { virtuals: true }
});

ProjectSchema.virtual("taskCount").get(function(){
    return this.tasks.length;
});

But I get the following error: TypeError: Cannot read property 'length' of undefined

Answer 1

Because you're excluding tasks, the virtual property cannot check the length of the property (as it's not defined).

Either include tasks, or include the length as a precomputed value of your schema. You could just use $inc on the field to change the length as new items are added to the array of tasks (or just set it directly).