[fpc-pascal] Built in Query language (or abuse)

[Adrian Veith]

L schrieb:
> A creative idea:
>
> --Another idea--
> Remember the "file of Record" innovation in pascal?
>
> Table of Record...
>
> A "file of record" allows one to remain strongly typed. What about a 
> "DatabaseTable of Record".
>
> var
>  // F: File Of DataRec;
>  DB: Table of DataRec; // aka a RelVar
>

We do something similar with our dbGonzales Database engine with 
standard pascal code. The tables are typed and have coresponding 
interface classes in oo-pascal. A table can hold any of the subclasses 
of its baseclass. If you access a "record" you get automatically the 
corresponding interface class and can use its methods.

A small demo to visualize this is here 
http://www.db-gonzales.de/download/GDemo.zip. The demo draws objects 
(circle, line, rectangle, ellips) stored in database and animates them 
(or adds new). If you start more instances of the demo, you can see what 
the other instance is doing with the objects.

If you look at the source of the demo, you can see, that it is also 
possible to write queries with pascal notation, which are already 
checked at compile time - but I must admit, that this a little bit clumsy.

        qy:= TGdfFilteredTableOp.Create(tb, [],
            TfComp.New(tb.BaseClass.FieldByName('ParentID'), 
TvVariant.Val(Null), [cvEQ]));

could also be written as:

        qy:= TGdfFilteredTableOp.Create(tb, [],
            TfComp.New(TPaintObject(tb.BaseInterfaceClass).ParentID, 
TvVariant.Val(Null), [cvEQ]));

and is the same as:

        qy:= TGdfFilteredTableOp.Create(tb, [],
            'ParentID is null' );

which is better readable.


So my opinion is - we don't need to change pascal to achieve things like 
that, it's already in the language

Cheers, Adrian.

p.s: The information on the webpage about the database is outdated - i 
haven't found the time to write a documentation.