Factory Girl- Create a Array Inside of Json

I am trying to create a json that contains an array with FactoryGirl. This is for unit testing a Rails controller that sends back a api.

From rails: how to unit test a json controller?, I would have:

describe "GET 'show'" do

  before(:each) do
    @company = Factory(:company)
    get 'show', :format => :json, :id => @company.id
  end

  it "should be successful" do
     response.should be_success
  end

  it "should return the correct company when correct id is passed" do
    body = JSON.parse(response.body)
    body["id"].should == @company.id
  end

end

but the issue I am having is trying to make the json with the array in Factory Girl:

FactoryGirl.define do
  factory :datacenter, class:Array do
    datacenters ["TX", "WA", "MI", "foo"]
  end
end

which the json would look like:

{"datacenters": "TX", "WA", "MI", "foo"]}