How to pluck a Backbone collection's attribute

Question

I want to create an array of specific attribute values from a Backbone collection.

var days = _.select(
    this.collection.models,
    function(model) {
        return model.attributes.type === 'session';
    }
);

days = _.pluck(days, 'attributes'),
days = _.pluck(days, 'date');

This works, but seems inefficient. Is there a way to accomplish the same thing without having to define days three times?

Answer 1

pluck is a convenience method that wraps map, and map is available directly on the collection, which should make this easier.

assuming you are trying to get the date attribute out of your models, you can do this:

days = this.collection.map(function(model){
  return model.get('date');
});

your select call is also available on the collection directly, as the filter method.

days = this.collection.filter(function(model){ 
  return model.attributes.type === 'session'; 
});

you can chain these two together, but it helps if you define methods separately:

var sessionFilter = function(model){
  return model.attributes.type === 'session'; 
};
var getDate = function(model){ return model.get('date'); }

days = this.collection.filter(sessionFilter).map(getDate);

this should return the results your looking for... or something close to this at least :)

Answer 2

I should have read the docs more carefully. In addition to the pluck method in underscore, backbone collections also have a pluck method.

http://documentcloud.github.com/backbone/#Collection-pluck

So, my solution would be

//Assumme 'collection' is a backbone collection
collection.pluck('date');

I still need to figure out how to best combine with filtering - perhaps using ideas from @Derick's answer, but this answers the meat of my own question.

Answer 3

I think this could work :

var days =
    _( this.collection.where({ type : "session" }))
    .chain()
    .pluck("attributes")
    .pluck("date")
    .value()

Slightly more elegant, but still close to unreadable, in my opinion.

Answer 4

Have the same question, and figured out a better solution. I might have just taken advantage of new features in underscore, since it's 2013 now :)

var days = _.invoke(this.collection.where({type : 'session'}), 'get', 'date');

Answer 5

This is functionally the same as Derick's answer using _.chain()

var days = _(this.collection.models)
            .chain()
            .filter(function(model){return model.attributes.type === 'session';})
            .pluck('date')
            .value();