table inheritance in rails

[Adam Akhtar-2]

Hello,

Suppose I have 2 models Human and Person and I want to inherit one from
another so that I could use 'human' model/migration as a generic builder
for smiliar objects as 'person', for instance this with the following
table structure:

human table:
id
weight
size

person table:
id
name
surname
address

Is it possible to make such table inheritance in rails to be able to
have 'base' model for those that are submodels with different columns?

Thanks in advance!
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[Conrad Taylor]

On Thu, Nov 5, 2009 at 4:23 PM, Aljaz Fajmut <rails-mailing-list@...> wrote:

Hello,

Suppose I have 2 models Human and Person and I want to inherit one from
another so that I could use 'human' model/migration as a generic builder
for smiliar objects as 'person', for instance this with the following
table structure:

human table:
id
weight
size

person table:
id
name
surname
address

Is it possible to make such table inheritance in rails to be able to
have 'base' model for those that are submodels with different columns?

Thanks in advance!

Yes, this is possible in Rails.  Thus, you can read section 19.4 of AWDwR 3rd edition.  It 

provides an excellent example of what one would do to achieve table inheritance in Rails.

Good luck,

-Conrad

 

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[Adam Akhtar-2]

Aljaz Fajmut wrote:
> Hello,
>
> Suppose I have 2 models Human and Person and I want to inherit one from
> another so that I could use 'human' model/migration as a generic builder
> for smiliar objects as 'person',

Your data modeling is faulty.  Human is not a sub- or superclass of
Person; rather, they're near synonyms.

> for instance this with the following
> table structure:
>
> human table:
> id
> weight
> size
>
> person table:
> id
> name
> surname
> address

Where's the inheritance here?  There are no fields in common except id.
What do you actually want to do?

>
> Is it possible to make such table inheritance in rails to be able to
> have 'base' model for those that are submodels with different columns?
>

Well, there's single-table inheritance (see the Rails docs), but it's
seldom a good idea...

> Thanks in advance!

Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen@...
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[Adam Akhtar-2]

It might not be the best example but I couldnt think of better one in
the moment in wrote this. I have a clear image of what I want to
implement.

Is this single or multi-table inheritance?

Thanks for help

Marnen Laibow-Koser wrote:

> Aljaz Fajmut wrote:
>> Hello,
>>
>> Suppose I have 2 models Human and Person and I want to inherit one from
>> another so that I could use 'human' model/migration as a generic builder
>> for smiliar objects as 'person',
>
> Your data modeling is faulty.  Human is not a sub- or superclass of
> Person; rather, they're near synonyms.
>
>> for instance this with the following
>> table structure:
>>
>> human table:
>> id
>> weight
>> size
>>
>> person table:
>> id
>> name
>> surname
>> address
>
> Where's the inheritance here?  There are no fields in common except id.
> What do you actually want to do?
>
>>
>> Is it possible to make such table inheritance in rails to be able to
>> have 'base' model for those that are submodels with different columns?
>>
>
> Well, there's single-table inheritance (see the Rails docs), but it's
> seldom a good idea...
>
>> Thanks in advance!
>
> Best,
> --
> Marnen Laibow-Koser
> http://www.marnen.org
> marnen@...

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[Adam Akhtar-2]

Aljaz Fajmut wrote:
> It might not be the best example but I couldnt think of better one in
> the moment in wrote this. I have a clear image of what I want to
> implement.

Then you should be able to explain it more clearly. :)

>
> Is this single or multi-table inheritance?

They're functionally more or less equivalent, although single-table
inheritance is rather a perversion of the relational model.  The choice
between the two usually seems to be made on practical grounds.
>
> Thanks for help
>
>

Best,
--
Marnen Laibow-Koser
http://www.marnen.org
marnen@...
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[Adam Akhtar-2]

Ok - I might be complicating too much.

Heres how I would do it with c++, not 100% sure what approach should I
take with rails on the database/model level.

If I want to have generic data-storage model Vehicle (which will hold
information the same information for all the vehicles - car, boat, etc.)

class vehicle holds:
color, max-speed, ..

class car:
color, max-speed, number of seats, ..

class boat:
color, max-speed, ... with some boat-specific columns

I'd just derive car from vehicle and use car-specific data columns
because similar operations will be called on it as on boat model.

As far as I understand RoR approach: if I have a lot of duplicated code
in my application, there must be some better solution for my problem.

What do you think?


Marnen Laibow-Koser wrote:

> Aljaz Fajmut wrote:
>> It might not be the best example but I couldnt think of better one in
>> the moment in wrote this. I have a clear image of what I want to
>> implement.
>
> Then you should be able to explain it more clearly. :)
>
>>
>> Is this single or multi-table inheritance?
>
> They're functionally more or less equivalent, although single-table
> inheritance is rather a perversion of the relational model.  The choice
> between the two usually seems to be made on practical grounds.
>>
>> Thanks for help
>>
>>
>
> Best,
> --
> Marnen Laibow-Koser
> http://www.marnen.org
> marnen@...

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[Conrad Taylor]

On Thu, Nov 5, 2009 at 5:44 PM, Aljaz Fajmut <rails-mailing-list@...> wrote:

It might not be the best example but I couldnt think of better one in
the moment in wrote this. I have a clear image of what I want to
implement.

Is this single or multi-table inheritance?

You can implement this as single table inheritance because a RDBMS

doesn't support multi-table inheritance (i.e.one cannot inherit the fields

of another table).  If this is what you're looking for, then you'll need an

OODB which represents things as object graphs instead of table rows.

Lastly, Marnen is correct in saying that Person and Human are synonyms

of one another and I recommend reading up on object oriented design.

Good luck,

-Conrad

 

Thanks for help

Marnen Laibow-Koser wrote:
> Aljaz Fajmut wrote:
>> Hello,
>>
>> Suppose I have 2 models Human and Person and I want to inherit one from
>> another so that I could use 'human' model/migration as a generic builder
>> for smiliar objects as 'person',
>
> Your data modeling is faulty.  Human is not a sub- or superclass of
> Person; rather, they're near synonyms.
>
>> for instance this with the following
>> table structure:
>>
>> human table:
>> id
>> weight
>> size
>>
>> person table:
>> id
>> name
>> surname
>> address
>
> Where's the inheritance here?  There are no fields in common except id.
> What do you actually want to do?
>
>>
>> Is it possible to make such table inheritance in rails to be able to
>> have 'base' model for those that are submodels with different columns?
>>
>
> Well, there's single-table inheritance (see the Rails docs), but it's
> seldom a good idea...
>
>> Thanks in advance!
>
> Best,
> --
> Marnen Laibow-Koser
> http://www.marnen.org
> marnen@...

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[Todd A. Jacobs-20]

On Fri, Nov 06, 2009 at 01:23:25AM +0100, Aljaz Fajmut wrote:


> human table: id weight size
> person table: id name surname address

If I understand your question correctly, you need to build the right
relationships. If you lived in the Star Trek universe, you might have:

    class Person < ActiveRecord::Base
        belongs_to :race
    end
    class Race < ActiveRecord::Base
        has_many :people
    end

where @person.race.name should return "human" if you've populated the
race table and created a foreign key in person named "race_id".

--
"Oh, look: rocks!"
        -- Doctor Who, "Destiny of the Daleks"


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