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Fairly new to backbone, so this is a really basic question. I have a Backbone collection passed into a function and I can prove that it has been passed and that the models in the collection have ids.

Here's how I'm setting the ids -

convertToMapObjects: (results)  =>
   objectList = new ObjectList()
    testObj = new TestObject()
      id = result.get("id")

And in another function ( accessed through making the model trigger an event) -

getIds: (objects) =>
ids = (object.id for object in objects) 

I think the issue may be because of how I'm iterating through the collection because when I tried doing

for object in objects

I saw two undefineds. Is this correct? If so, why can't I use a for loop to go through a backbone collection? Also, is there a way I could do so?

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3 Answers

up vote 18 down vote accepted

A Backbone collection is not an array so for ... in won't produce the results you're expecting. You want to look at the collection's models property if you want to use a simple loop.

However, Backbone collections have various Underscore methods mixed in:

Underscore Methods (28)


  • forEach (each)
  • ...

So you can use map or pluck if you'd like to avoid accessing the models property:

ids = objects.map (m) -> m.id
ids = objects.pluck 'id'

The pluck method is, more or less, just a special case of map but collections implement a native version rather than using the Underscore version so that they can pluck model attributes rather than simple object properties.

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this answered precisely my question - thank you –  praks5432 Aug 2 '12 at 20:46
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You want to loop over the models property of the collection, not the collection object itself.

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for object in object.models

This will give you a model in the collection

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