Re: [gobo-eiffel-develop] DS_BILINEAR and DS_BILINEAR_CURSOR

[Eric B. <er...@go...>]

Uwe Sander wrote:
> 
> The problem is slightly different. I am dealing with databases like MySQL,
> PostgreSQL and so on and these systems do not deliver objects to my access
> classes but simply values, so I have decided to assign exactly one record
> object to one cursor and fill the record with values according to its
> position. So I have the problem that two cursors never reference the same
> record (item) even if they point to the same position in the underlying
> dataset.

You may have noticed that the postcondition is only in
DS_LINEAR.cursor_is_first and not in DS_LINEAR_CURSOR.is_first.
The reason why it is in `cursor_is_first' is to guarantee a
precise semantics for the internal cursor. However a cursor
may decide to traverse the container in reverse order or skip
some items, that's why the postcondition on DS_LINEAR_CURSOR.is_first
is not as strict as on `cursor_is_first'. Of course if you
want to have a cursor with a different semantics than the
internal cursor of the container, you need to redefine features
in the cursor class such as `is_first' and provide a different
implementation from those provided by DS_LINEAR.cursor_*.
So with your internal cursor the postcondition of `cursor_is_first'
should work (provided that the routine `first' in your container
returns the record object associated with your internal cursor),
and for external cursor you would have to redefine `is_first'
to avoid calling `cursor_is_first'. Or do you have separate record
objects for `first' and `last', different from the record object
associated with the internal cursor? In this latter case my solution
would not work.

> Hmm, while I am writing this, I have to realize that it is _not_ sufficient
> for database access classes to compare values to decide if a certain record
> is really the first one of a set of data :-( Even if SQL rules forbid
> duplicate rows within a table, it is still possible to write SELECTs which
> yield multiple identical rows.

This is not a problem which only occurs with DBs. For example
in a list I can insert several times the some object.

-- 
Eric Bezault
mailto:er...@go...
http://www.gobosoft.com