Retrieving an object in a jbuilder partial return 'nil' unless it is wrapped in an array

I am trying to use a jbuilder to render a json response for a TypeCar object in my response

class TypeCar < ActiveRecord::Base
  attr_accessible :name

  belongs_to :car_alias, foreign_key: :name, primary_key: :name, counter_cache: true
  has_one :main_car, through: :car_alias, foreign_key: :name, primary_key: :name

end

In the json for the TypeCar partial, "views/api/type_cars/_type_cars.json.jbuilder",

json.extract! type_car, :name

json.main_car Array.wrap(type_car.main_car) do |main_car|
    json.partial! "api/main_cars/main_cars", main_car: main_car
end

And the main car partial in "api/main_cars/_main_cars.json.jbuilder"

json.extract! main_car, :more_info, :name, :id

The view that uses the _type_car partial, returns the following

{
name: test,
main_car: [
{
more_info: "Lorem Ipsum",
name: "New",
id: 198
}
]

I am guessing that the for main car attribute is a hash in a single object array because of wrapping up the array. But I would like the remove this as it is one more extra level of nesting that is not needed. So I would just prefer a plain dictionary instead of a hash.

The reason I had to wrap the object in an array and pass it to an enumerable was because nothing else seemed to work.

This...

json.extract! type_car, :name

json.main_car do 
    json.partial! "api/main_cars/main_cars", main_car: type_car.main_car
end

Gives this error...

undefined method `more_info' for nil:NilClass

And this:

json.extract! type_car, :name

json.main_car type_car.main_car do |main_car|
    json.partial! "api/main_cars/main_cars", main_car:main_car
end

Gives this error:

undefined method `map' for #<MainCar:>

Even trying stuff like type_car.main_car.id within the type car partial returns the nil problem.

Really appreciate all the help and guidance, thanks!