Fawzi Mohamed wrote:
On 4-apr-11, at 02:01, Piotr Szturmaj wrote:
Fawzi Mohamed wrote:
[...]
I think that your responses are very relevant, as it seems to me that
your work is nice, and I find that if a GSoC is done in that direction
it should definitely work together with the good work that is already
done, let's don't create multiple competing projects if people are
willing to work together.
I'm ready to cooperate :)
great :)
* support for static and dynamic types.
how access of dynamic and static types differs, should be as little as
possible, and definitely the access one uses for dynamic types should
work without changes on static types
If you mean statically or dynamically typed data row then I can say my
DBRow support both.
yes but as I said I find the support for dynamic data rows weak.
I've just added row["column"] bracket syntax for dynamic rows.
excellent, ideally that should work also for untyped, because one wants
to be able to switch to a typed Row without needing to change its code
I used to think the same, but currently this is technically impossible.
When I started working on this I wanted one common interface, but tuples
use static indexing to their fields. You can't write such code:
Tuple!(int, string) t;
int index = 1;
// try access string field:
t[index] = "abc"; // error
but this works:
t[1] = "abc"; // ok
This problem also applies to structs (FieldTypeTuple). To overcome that
we need to split opIndex to compile-time one and run-time one (add
static opIndex).
> (and it should work exactly the same, so the typed rows will need to
> wrap things in Variants when using that interface).
Yes, I tried hard to do it. It worked, but it broke Tuple index access -
it was hidden by opIndex.
* class or struct for row object
I'm using struct, because I think row received from database is a
value type rather than reference. If one selects rows from one table
then yes, it is possible to do some referencing based on primary key,
but anyway I think updates should be done explicitly, because row
could be deleted in the meantime. In more complex queries, not all of
selected rows are materialized, i.e. they may be from computed
columns, view columns, aggregate functions and so on. Allocation
overhead is also lower for structs.
* support for table specific classes?
Table specific classes may be written by user and somehow wrap
underlying row type.
well with the current approach it is ugly because your calls would be
another type, thus either you remove all typing or you can't have
generic functions, accepting rows, everything has to be a template,
looping on a table or a row you always need a template.
Could you elaborate? I don't know what do you mean.
Well I am not totally sure either, having the row handle better the
dynamic case i already a nice step forward, I still fear that we will
have problems with the ORM level, I am not 100% sure, that is the reason
I would like to try to flesh out the ORM level a bit more.
I would likethat one can loop on all the tables and for each one get the
either the generic or the specialized object depending on what is needed.
If one wants to have business logic in the specialized object it should
be difficult to bypass them.
Well, it should be possible right now:
struct MyData
{
int a;
int b;
int multiply()
{
return a * b;
}
}
auto cmd = new PGConnection(conn, "SELECT a, b FROM numbers")
auto result = cmd.executeQuery!MyData;
foreach (row; result)
writeln(row.multiply);
> Maybe I am asking too much and the ORM level should never expose the
> rows directly, because if we use structs we cannot have a common type
> representing a generic row of a DB which might be specialized or not
> (without major hacking).
ORM level may of course expose rows. It should be an additional level of
abstraction built on top of SQL api. So one can mix SQL and ORM interfaces.
In regards to common type, it's currently impossible to wrap a Tuple or
struct and use [index] access to fields. No matter if we use struct or not.
* reference to description of the table (to be able to get also
dynamic
types by column name, but avoid using too much memory for the
structure)
My PostgreSQL client already supports that. Class PGCommand has member
"fields", which contain information about returned columns. You can
even check what columns will be returned from a query, before actually
executing it.
ok that is nice, and my point is that the type that the user sees by
default should automatically take advantage of that
* Nice to define table structure, and what happens if the db has
another
structure.
This is a problem for ORM, but at first, we need standard query API.
I am not so sure about this, yes these (also classes for tables) are
part of the ORM, but the normal users will more often be at the ORM
level I think, and how exactly we want the things look like that the
object level can influence the choice of the best low level interface.
A "defined" DBRow or static one, if used on result which has inequal
number of columns or their types aren't convertible to row fields then
it's an error. But, if someone uses a static fields, he should also
take care that the query result is consistent with those fields.
For example doe we want lazy loading of an object from the db? if yes
how we represent it with current Rows objects?
Could you post an example of lazy loading of an object?
* you want to support only access or also db creation and
modification?
First, I'm preparing base "traditional" API. Then I want to write
simple object-relational mapping. I've already written some code that
generated CREATE TABLE for structs at compile time. Static typing of
row fields is very helpful here.
Very good I think that working on getting the API right there and having
it nice to use is important.
Maybe you are right and the current DBRow is indeed the best
abstraction, but I am not yet 100% sure, to me it looks like it isn't
the best end user abstraction (but it might be an excellent low level
object)
I should state here, that end-user usability is very important to me.
I should also clarify that my code isn't completely finished and of
course it is a subject to change. Any suggestions and critics are
welcome :)
very good :)
