Rails - Apr 2009 - help active record modeling.

Hi.

I have difficulties to model active record.
So please help my trouble.

Let''s suppose there are nations which have castles at least 1,
and there are 3 types of castles.

So I will make these tables.
NationsTable,
CastlesTable have common attributes of 3 types of castle.
    CastlesATable, CastlesBTable, CastlesCTable each have its unique
attributes.

In this case how can I represent this DB to AR?
I think
class Nation < ActiveRecord::Base
   has_many :castles
end
class Castle < ActiveRecord::Base
  belongs_to :Nations
end

but I''m not sure of what will be coded to these models.

class CastleA < ActiveRecord::Base
end
class CastleB < ActiveRecord::Base
end
class CastleB < ActiveRecord::Base
end

please help me.

and second question is
when nation deleted, CastleA, CastleB have to be deleted but not for
the castleC type.
how can I do?

thanks for reading.

2009/4/29 serenobs <serenobs-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
>
> Hi.
>
> I have difficulties to model active record.
> So please help my trouble.
>
> Let''s suppose there are nations which have castles at least 1,
> and there are 3 types of castles.
>
> So I will make these tables.
> NationsTable,
> CastlesTable have common attributes of 3 types of castle.
>    CastlesATable, CastlesBTable, CastlesCTable each have its unique
> attributes.
>
Easier to call your tables: ''nations'' for
''Nation'' model and ''castles'' for
''Castle'' model ... etc

>
> In this case how can I represent this DB to AR?
> I think
> class Nation < ActiveRecord::Base
>   has_many :castles
> end
> class Castle < ActiveRecord::Base
>  belongs_to :Nations
> end
>
Using the naming convention I suggested above you''d say:
     belongs_to :nation

After all, a castle can only belong to one nation at a time right?

> but I''m not sure of what will be coded to these models.
>
> class CastleA < ActiveRecord::Base
> end
> class CastleB < ActiveRecord::Base
> end
> class CastleB < ActiveRecord::Base
> end
>
> please help me.
>
You could use single table inheritance (STI).
1)  all castles are stored in one table (''castles'') which has
all the
required attributes for all types of castles.
2) in app/models you''ll have your Castle model and then you''ll
have a
CastleA,B and C models like above but you now inherit from Castle:
    class CastleA < Castle
    end
3) You include a ''type'' field in ''castles''
table which is a string and is
the same as the class name  of the castle. (ie for an A-type it would be
''CastleA'')

This is a tidy way to handle your castles as everything is in one table.  It
does mean that not all attributes will be used by a given castle instance if
you have specialised attributes; these will probably be null so I don''t
think the wastage will be much.

>
> and second question is
> when nation deleted, CastleA, CastleB have to be deleted but not for
> the castleC type.
> how can I do?
>
I''m never good at this stuff.  But you could do it with a callback in
your
Nation model:

class Nation < ActiveRecord::Base
  has_many :castles
  before_destroy do |nation|
    Castle.destroy_all "nation_id=#{nation.id} and
type!=''#{CastleC}''"
  end
end

There''s probably a sexier way to do that.




-- 
Daniel Bush

http://blog.web17.com.au
http://github.com/danielbush/sifs/tree/master
http://github.com/danielbush

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