rails find_by_object - ruby-on-rails

Question

I know the rails method find_by_name but what if instead of a name I have an object? Assuming I have #car and #brand objects. How can I search for all cars that have a specific brand object.
Something like #cars = Car.find_by_brand(#brand)
I have tried #cars = #brand.cars but that only seems to pull one parent model.
Edit - More info on code
Car controller:
has_and_belongs_to_many :brands
Brands model:
attr_accessible :name
has_and_belongs_to_many :cars
Brands Controller
def create
#car = Car.find(params[:car_id])
#brand = Brand.create
#brand.assign_attributes({ :name => params[:brand][:brands][:name] })
#brand.cars << #car
if #brand.save
redirect_to #car
else
flash[:notice] = "Error!"
redirect_to #car
end
end
def findcars
#brand = Brand.find_by_name(params[:brand_name])
#cars = #brand.cars
end
View
<%= link_to brand.name, findcars_car_brand_path(#car, brand_name: brand.name), method: "get" %>
Routes
resources :cars do
resources :brands do
member { get :findcars }
end
end
Tables - There is no model for the join table
brands (name:string)
cars (name:string)
brands_cars (brand_id:integer, car_id:integer)

Answer 1

Let's look at findcars first. .joins(:brands) is key as it allows you to narrow the query via brand conditions ('brands.attribute' => brand_value), while building Car objects.
The SQL will look something like this:
SELECT "cars".* FROM "cars" INNER JOIN "cars_brands" ON "cars_brands"."car_id" = "cars"."id" INNER JOIN "brands" ON "brands"."id" = "cars_brands"."brand_id" WHERE "brands"."id" = 1
and the implementation like this:
# If all you care about is the #cars - as the method name indicates
def findcars
#cars = Car.joins(:brands).where('brands.name' => params[:brand_name])
end
# If you need both variables
def findcars
#brand = Brand.find_by_name(params[:brand_name])
#cars = Car.joins(:brands).where('brands.id' => #brand)
end
Without the joins, your sql looks like this:
SELECT "cars".* FROM "cars" WHERE "brands"."id" = 1
This will break as 'brands' means nothing in this context. You need to give rails a little more detail so it can build the query properly.
Now the create method:
# #brand = Brand.create
# #brand.assign_attributes({ :name => params[:brand][:brands][:name] })
# params[:brand][:brands][:name] indicates something fishy with your form
# since this form is in the context of Brand, params[:name] should suffice
#brand = Brand.create(:name => params[:brand][:brands][:name])
Does redirect_to #car mean that the form is a Car-centric form? Using *form_for* in the correct context will simplify the object creation.
For 2 great overviews, check out:
http://guides.rubyonrails.org/active_record_querying.html
http://guides.rubyonrails.org/association_basics.html