Rails associations

[John Blaze]

Hi,

I am new to RoR and have been pulling my hair out trying to figure out
associations.

Lets say I have 2 tables: vehicles and vehicle_types. ONE vehicle HAS
ONE vehicle_type. Therefore, in my opinion, vehicle_type belongs_to
vehicle.

Hence, vehicle_types will contain the vehicle foreign key. To me this is
already weird. I would like vehicle_types to be a table containing a
column that lists types of vehicles- "truck", "car", "bus", etc. It does
not need to know what vehicle is associated with it. The vehicles table
needs to know what vehicle_type it is.

I would like to use the vehicle_types table to populate a Select Menu on
a form. If I create the associations as suggested by RoR, with the
foreign key going on the belongs_to table (vehicle_types), the Select
Menu would be populated with duplicate vehicle_types. EG:

vehicle_id vehicle_type_id type
1 1 car
2 2 truck
3 3 car

Does this make sense?

Like I said I have spent days reading over the associations again and
again but I just can't figure it out. Any help would be greatly
appreciated.

Thanks.

--
Posted via http://www.ruby-forum.com/.

[Colin Law]

On 1 August 2012 06:04, John Blaze <li...@ruby-forum.com> wrote:
> Hi,
>
> I am new to RoR and have been pulling my hair out trying to figure out
> associations.
>
> Lets say I have 2 tables: vehicles and vehicle_types. ONE vehicle HAS
> ONE vehicle_type. Therefore, in my opinion, vehicle_type belongs_to
> vehicle.

No, the names of associations are confusing at times. In your case a
vehicle type can be associated with many vehicles, therefore
vehicle_type has_many vehicles. A vehicle is only of one type, so
vehicle belongs_to vehicle_type, and it is the vehicle that has the
vehicle_type_id.

Colin

>
> Hence, vehicle_types will contain the vehicle foreign key. To me this is
> already weird. I would like vehicle_types to be a table containing a
> column that lists types of vehicles- "truck", "car", "bus", etc. It does
> not need to know what vehicle is associated with it. The vehicles table
> needs to know what vehicle_type it is.
>
> I would like to use the vehicle_types table to populate a Select Menu on
> a form. If I create the associations as suggested by RoR, with the
> foreign key going on the belongs_to table (vehicle_types), the Select
> Menu would be populated with duplicate vehicle_types. EG:
>
> vehicle_id vehicle_type_id type
> 1 1 car
> 2 2 truck
> 3 3 car
>
> Does this make sense?
>
> Like I said I have spent days reading over the associations again and
> again but I just can't figure it out. Any help would be greatly
> appreciated.
>
> Thanks.
>
> --
> Posted via http://www.ruby-forum.com/.
>

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[John Blaze]

Thank you so much for your reply Colin.

I believe you are spot on with your answer and what you suggest will
work and is the correct solution.

For sake of my sanity though, is this solution more of a Rails
convention or have I been getting standard database design confused in
my head for a number of years? To my previous understanding I still see
it as one vehicle has one vehicle_type. Almost like vehicle_type is a
child table to the parent vehicle. Is this wrong thinking for all
database design or just Rails conventions?

[Colin Law]

vehicle_type cannot be like a child to the vehicle, because it is
associated with multiple vehicles, how can it be a child of multiple
parents? It is the other way round if you want to think of child and
parent. A parent (vehicle type) has many children (vehicles), but a
child (vehicle) belongs to a single parent. If you are thinking in
conventional database design just remember that it is the table that
includes the foreign key (vehicle.vehicle_type_id in this case) that
must specify the belongs_to association.

Colin

>
> --
> Posted via http://www.ruby-forum.com/.
>

[John Blaze]

Once again, fantastic help Colin. Great explanation and it is all
becoming much clearer for me now. Your help was very much appreciated.

[Matt Jones]

On Wednesday, 1 August 2012 03:55:31 UTC-4, Ruby-Forum.com User wrote:

Thank you so much for your reply Colin.

I believe you are spot on with your answer and what you suggest will
work and is the correct solution.

For sake of my sanity though, is this solution more of a Rails
convention or have I been getting standard database design confused in
my head for a number of years? To my previous understanding I still see
it as one vehicle has one vehicle_type.

Sometimes the names can have extra baggage that makes it confusing - for example, it seems clearer to say "one vehicle belongs to a single vehicle category".

--Matt Jones

 

[John Blaze]

Ok, the associations are getting the better of me again. My models for
Business, VehicleType and Vehicle.


class Business < ActiveRecord::Base
has_many :vehicles
end

class VehicleType < ActiveRecord::Base
has_many :vehicles
end


class Vehicle < ActiveRecord::Base
belongs_to :business
belongs_to :vehicle_type
end

Now, my problem is in creating a new vehicle. I can create a Business
and VehicleType just fine, but when I try to create a Vehicle I run into
problems with the foreign keys.

If I try:
first_vehicle = Vehicle.new(:veh_name => "test")
#then I try adding the new vehicle to my existing business
business.vehicles << first_vehicle

it generates an error that the foreign key for the vehicle_types table
cannot be null. If I try to append the vehicle to the VehicleType first
I get an error that the business_id cannot be null.

Any ideas here?

[Andrew Vit]

It sounds like you're running up against database foreign key constraints: your model needs to have those id attributes filled in before you can save it. (This is often caught with validations at the rails application layer too, with `validates :vehicle_type, presence: true`)

This is one possible approach:

business = Business.first

car = VehicleType.find_by_name("car")

first_vehicle = Vehicle.new

first_vehicle.business = business

first_vehicle.vehicle_type = car

first_vehicle.save

Or another way:

first_vehicle = Vehicle.create do |v|

  v.business = business

  v.vehicle_type = car

end

Or:

first_vehicle = business.vehicles.build(vehicle_type: car)

first_vehicle.save

Or again:

first_vehicle = business.vehicles.create(vehicle_type: car)

[John Blaze]

Wow. Amazing response Andrew. I will report back if I have any trouble
implementing this.

Is there a guide anywhere for best practices with Rails? You have given
some great examples but as a newbie I would like to commit to my memory
those practices that are highly recommended.