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).
-
all castles are stored in one table (‘castles’) which has all the required attributes for all types of castles.
-
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
-
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.