Re: Object-relational impedence

From: Yagotta B. Kidding <>
Date: Sun, 9 Mar 2008 18:13:51 +0100 (CET)
Message-ID: <Xns9A5C86B49BE68vdghher_at_194.177.96.26>

S Perryman <> wrote in news:fr0hft$8gf$

> Yagotta B. Kidding wrote:

>> S Perryman <> wrote in news:fqpj0o$lke$

> TN>In the other message
> TN>you dismissed projection as being covered by the concept of
> TN>subclassing.
> SP>No. Projection is covered by *type substitutability* .
> TN>Can you please be more specific? If we remove some [data]
> TN>attributes, does it mean the resulting "entity" is a subclass.
>>>No, merely that the resulting entity is now deemed to be of
>>>another type, substitutable with the original type.

>>>type T
>>>    x, y, z
>>>Set<T> ts ;
>>>Set< type { x, y } > ps = { e IN ts : e.x > 123 } ;

>>>The elements of ps are effectively projections of the elements in ts.

>> Hold on, you have a value of type Set<T> assigned to a variable of
>> type Set<type{x,y}>.

> Basic type substitutability (structural equivalence) .
> The { x, y } type has properties in common with T (the x/y properties)
> . Therefore the assignment is legal.
> So the assignment is a projection is of a type (x,y,z) to a type (x,y)

I think you are confused at least on two counts.

In any OO langauage I am familiar wth, you can assign a subtype value to a supertype variable. However, you did not establish any subtype/supertype relationship in your example, therefore, the assignment is illegal.

Even when you establish such relationship, no projection really takes place, the information is still there, it's just hidden, as can easily be seen by casting back to the original subtype.

> Regards,
> Steven Perryman
Received on Sun Mar 09 2008 - 18:13:51 CET

Original text of this message