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Anton Aylward

2007-Feb-03 14:22 UTC

Alias of Table?

I''m attempting to convert a legacy application.

I need to do what amounts to a "alias filter" on a table.
The table - call it ''Assets'' - has many name-value pairs as
its public data.
It is shared  by many other tables, so the key is

     parent_id, parent_class

This is a one-way association: "Assets of X", where "X" can
be another table.

In base SQL this is easy - set up an alias

My Question- ow to do this in Rails:
Is there a way to filter for a ''parent'' class so this can be
shared?

Class OneThing < AR::B
   has_many	:assets # where asset.parent_class = ''OneThing''


Class AnotherThing < AR::B
   has_many	:assets # where asset.parent_class =
''AnotherThing''


So how to make this ''automated'' for the CRUD and how to define
the ''Setting''
class?  Can ''Asset'' somehow belong_to more than one model?

I''ve no problem drawing the E-R diagram :-)
or thinking what the explicit and long winded SQL would be,
but I don''t have the Rail experience to see this.


I can see docco on ''has_and_belongs_to_many'' using an
intermediate join
table, but this is a one way association.


Thanks
/a
-- 
A seminar on time-travel will be held two weeks ago.

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Josh Susser

2007-Feb-04 17:06 UTC

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Re: Alias of Table?

Anton Aylward wrote:> I''m attempting to convert a legacy application.
> 
> I need to do what amounts to a "alias filter" on a table.
> The table - call it ''Assets'' - has many name-value pairs
as its public
> data.
> It is shared  by many other tables, so the key is
> 
>      parent_id, parent_class
> 
> This is a one-way association: "Assets of X", where "X"
can be another
> table.
> 
> In base SQL this is easy - set up an alias
> 
> My Question- ow to do this in Rails:
> Is there a way to filter for a ''parent'' class so this can
be shared?
> 
> Class OneThing < AR::B
>    has_many  :assets # where asset.parent_class =
''OneThing''
> 
> 
> Class AnotherThing < AR::B
>    has_many  :assets # where asset.parent_class =
''AnotherThing''
> 
> 
> So how to make this ''automated'' for the CRUD and how to
define the
> ''Setting''
> class?  Can ''Asset'' somehow belong_to more than one
model?
> 
> I''ve no problem drawing the E-R diagram :-)
> or thinking what the explicit and long winded SQL would be,
> but I don''t have the Rail experience to see this.
Rails does this using something called a polymorphic association. It''s 
the only explicit support in Rails for composite foreign keys. Your 
foreign key is composed of the values parent_id and parent_type. The 
class name of the parent model is stored as a string in parent_type.

You set this up using the :polymorphic => true option on the :parent 
association

  belongs_to :parent, :polymorphic => true

Then in the various classes, use :as => :parent on the has_many

  has_many :assets, :as => :parent

There are a few limitations related to this being a one-way association. 
You can''t do eager loading of parents, and has_many :through needs some
help to be useful. Otherwise it works pretty well.

--
Josh Susser
http://blog.hasmanythrough.com/

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

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Anton Aylward

2007-Feb-04 23:45 UTC

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Re: Alias of Table?

Josh Susser said the following on 02/04/2007 12:06 PM:> Rails does this using something called a polymorphic association.
It''s
> the only explicit support in Rails for composite foreign keys. Your 
> foreign key is composed of the values parent_id and parent_type. The 
> class name of the parent model is stored as a string in parent_type.
> 
> You set this up using the :polymorphic => true option on the :parent 
> association
> 
>   belongs_to :parent, :polymorphic => true
> 
> Then in the various classes, use :as => :parent on the has_many
> 
>   has_many :assets, :as => :parent
That seems odd to me, but what do I know. If I knew I wouldn''t be
asking :-)

So this would work:

class Asset < AR:B
   belongs_to :parent, :polymorphic => true

   def set(name, value)
   end

   def value_of(name)
   end
end

class Person < AR:B
   has_many :assets, :as => :parent
end

class Corporation < AR:B
   has_many :assets, :as => :parent
end

class Bank < Corporation
   has_many :assets, :as => :parent
end

class Government < AR:B
   has_many :assets, :as => :parent
end

class CIA < AR:B
   has_many :assets, :as => :parent
end

class Harry <AR:B
   has_many :assets, :as => :parent

   def make_my_asset(date, punkname)
       assetname = date.to_string
       ....
   end
end


> There are a few limitations related to this being a one-way association. 
That''s OK, I suppose, so long as you want to know the assets belonging
to
something and not who owns a particular asset.
> You can''t do eager loading of parents, and has_many :through needs
some
> help to be useful. Otherwise it works pretty well.
Help?  useful?
By ''help'' do you mean  if one of the parents needed to create
or delete an
asset?  Or does Rails ''magically'' fill in the parent_id and
parent_class
somehow.

Would it help if these parents were a subclass of an AssetOwner class?

-- 
Authors (and perhaps columnists) eventually rise to the top of whatever
depths they were once able to plumb.
		-- Stanley Kaufman

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Rails - Feb 2007 - Alias of Table?

Alias of Table?

Re: Alias of Table?

Re: Alias of Table?