Add workflow version to classifications

Closes #254
Probably not. I did a `rails g migration` and it added the index
automatically. I'll remove it.
On Nov 5, 2014 8:30 AM, "Campbell Allen" <notifications@github.com> wrote:

> In db/schema.rb:
>
> > @@ -46,6 +47,7 @@
> >    add_index "classifications", ["user_group_id"], name: "index_classifications_on_user_group_id", using: :btree
> >    add_index "classifications", ["user_id"], name: "index_classifications_on_user_id", using: :btree
> >    add_index "classifications", ["workflow_id"], name: "index_classifications_on_workflow_id", using: :btree
> > +  add_index "classifications", ["workflow_version_id"], name: "index_classifications_on_workflow_version_id", using: :btree
>
> Do we think our query paths will use this index? My assumption is the
> resource will be returned and the version lookup will be performed with the
> PK id versus the reverse lookup via FK index from versions.
>
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> <https://github.com/zooniverse/Panoptes/pull/259/files#r19875648>.
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