how to do JSON return in this format?

gistfile1.txt

I have a model that looks like:

# == Schema Information
#
# Table name: vendors
#
#  id                 :integer          not null, primary key
#  company_name       :string
#  category           :string
#

in my index action (of my API) I want to return Vendor.all but the data needs to be re-organized.  Here's an example:

for this active record set:
[
  {id: 1, company_name: 'co1', category: 'cat1'},
  {id: 1, company_name: 'co2', category: 'cat1'},
  {id: 1, company_name: 'co3', category: 'cat2'},
  {id: 1, company_name: 'co4', category: 'cat2'},
  {id: 1, company_name: 'co5', category: 'cat2'}
]

I want to return something like this:

[
   {'cat1' => [
                {company_name: 'co1'}, {company_name: 'co2'}
              ]
    },
    {'cat2' => [
                {company_name: 'co3'}, {company_name: 'co4'},{company_name: 'co5'}
               ]
    }
]

how can I do that?