Question

I’m working on a Hobo app trying to tie together a few models properly.

Activity objects have many Page children. They also have many DataSet children.

Page objects have several different kinds of children. We’ll talk about Widget children, but there are several types with the same issue. An instance of a Widget belongs to a Page but also has a belongs_to relationship with a DataSet. Here’s the important point: the DataSet must belong to the containing Activity. So for any given @widget:

@widget.page.activity === @widget.data_set.activity

It’s easy enough to enforce this constraint in the model with a validation on save. The trick is presenting, within the Widget‘s form, a select menu of available DataSets which only contains DataSets for the current Activity

I was able to get this working for existing objects using a tag like this:

<data_set-tag: options="&DataSet.activity_is(&this.page.activity)" />

However, for a new Widget, this fails messily, because either &this or &this.page is not yet set. Even for a route which contains the page ID, like /pages/:page_id/widgets/new, I’m not really able to get an Activity to scope the list of DataSets with.

If this was proper Rails, I’d get in to the relevant controller method and make the Activity available to the view as @activity or something of the sort, but in Hobo the controllers seems to be 95% Magic™ and I don’t know where to start. The knowledge of which Activity is current must be in there somewhere; how do I get it out?

This is Hobo 1.3.x on Rails 3.0.x.

---

ETA: The code producing the errors is in the form tag for Widget, like so:

<extend tag="form" for="Widget">
  <old-form merge>
    <field-list: fields="&this.field_order">
      <data_set-tag: options="&DataSet.activity_is(&this.page.activity)" />
    </field-list>
  </old-form>
</extend>

As I said above, this works for editing existing Widgets, but not new Widgets; the error is undefined method 'page' for nil:NilClass. Bryan Larsen’s answer seems to suggest that &this.page should not be null.

Answer 1

In the end, it turned out to be syntax. Instead of

<data_set-tag: options="&DataSet.activity_is(&this.page.activity)" />

I needed

<data_set-tag: options="&DataSet.activity_is(@this.page.activity)" />

(note the @).

We actually made this into a helper method, so the final code is

<data_set-tag: options="&DataSet.activity_is(activity_for(@this))" />