Meteor 0.8.0, UI Expected valid attribute name

Question

What causes this? I checked my data contexts and this actually has the appropriate data being passed in, and this worked in the previous version of Meteor.

Exception in Meteor UI: Error: Expected valid attribute name, '', null, or object

HTML:

    <select class='form-control' id='region' name="region" value="{{region}}">
      {{#each regions}}
        <option class="controls" value='{{_id}}' {{curRegion ../regionId}}>{{title}}</option>
      {{/each}}
    </select>

helper function:

Template.labEdit.helpers
  curRegion:  (region) ->
    console.log "Region: ", region, this
    return region is this._id && 'selected'

Full stack trace:

Exception in Meteor UI: Error: Expected valid attribute name, '', null, or object
    at Object.Spacebars.attrMustache (http://localhost:3000/packages/spacebars.js?5d478ab1c940b6f5a88f78b8adc81a47f022da77:137:11)
    at HTML.OPTION.$dynamic (http://localhost:3000/client/views/labs/template.labEdit.js?b7b4fc16112efc0d900b152a545a4279764a2728:61:26)
    at Object.HTML.evaluateAttributes (http://localhost:3000/packages/htmljs.js?697b0dd0fbdd1f8984dffa3225121a9b7d0b8609:347:21)
    at http://localhost:3000/packages/ui.js?b523ef986d3d39671bcb40319d0df8982acacfe8:2351:28
    at callWithNoYieldsAllowed (http://localhost:3000/packages/deps.js?7afb832ce6e6c89421fa70dc066201f16f9b9105:74:5)
    at _.extend._compute (http://localhost:3000/packages/deps.js?7afb832ce6e6c89421fa70dc066201f16f9b9105:212:7)
    at new Deps.Computation (http://localhost:3000/packages/deps.js?7afb832ce6e6c89421fa70dc066201f16f9b9105:144:10)
    at Object._.extend.autorun (http://localhost:3000/packages/deps.js?7afb832ce6e6c89421fa70dc066201f16f9b9105:361:13)
    at materialize (http://localhost:3000/packages/ui.js?b523ef986d3d39671bcb40319d0df8982acacfe8:2346:30)
    at materialize (http://localhost:3000/packages/ui.js?b523ef986d3d39671bcb40319d0df8982acacfe8:2293:7) 

and this is the section of compiled blaze that actually throws the error:

UI.block(function() {
    var self = this;
    return [ "\n                ", HTML.OPTION({
      "class": "controls",
      $dynamic: [ function() {
        return Spacebars.attrMustache(self.lookup("state"), Spacebars.dot(self.lookup(".."), "status"));
      } ]
    }, function() {
      return Spacebars.mustache(self.lookup("."));
    }), "\n              " ];
  })), "\n            "), "\n          "), "\n          ", HTML.DIV({
    "class": "col-md-4"
  }, "\n            ", Spacebars.With(function() {

Answer 1

The way to use valueless attributes like selected has changed in Blaze.

So you could replace your curRegion helper with an isRegion helper:

Template.labEdit.helpers
  isRegion: (region) -> @_id is region

And change the HTML line to this:

<option class="controls" value='{{_id}}' selected={{isRegion ../regionId}}>{{title}}</option>

In my opinion it also makes the helper function and the helper call much more readable (and flexible).