[web-devel] (Yesod) PersistEntity Key types

Rick Richardson rick.richardson at gmail.com
Sun Apr 3 16:44:40 CEST 2011

On Sun, Apr 3, 2011 at 10:35 AM, Michael Snoyman <michael at snoyman.com>wrote:

> As a short term solution, I think you're right. However, I think we might
> want to consider a slightly more sophisticated approach... I say consider
> specifically, because I'm really not certain that what I'm saying is a good
> idea.
> I think there's basically three problems with the approach of using a
> PersistValue inside a PersistKey:
> 1) It will probably hurt performance, since we'll need to do more checking.
> 2) The code becomes a bit more fragile. By adding this sum type to the mix,
> we're adding necessity for a number of checks that can fail.
> 3) There's nothing stopping you from inserting a value into one database
> (say, SQLite), getting a key, and then looking up in MongoDB. (Not that this
> is a flaw in the current approach as well.)

I agree on the first two points. I think the principle behind 3 is actually
a feature.

> So here's the idea: each database backend will have an associated type for
> its key datatype. Then, instead of having:
>     data Key entity = Int64
> we'll have
>     data Key entity backend = BackendKey backend
> (ignoring all the newtype wrappers). I think this should solve both the
> issue you raise about MongoDB, and the three points I mention above.
> However, I'm still concerned that it might lead to difficult-to-follow code.
> There's really only one way to find out, but I just wanted to bounce the
> idea around before diving in.

I am going to negate my previous email. After thinking on this a bit. I
think that setting they key type at the PersistEntity is the correct
This would be the only way to handle keys as strings or composite keys (if
we want to support either)

If we want to restrict all Yesod databases to a single pre-defined key type
per database, then I will implement your solution.  If we want to be
flexible (for legacy database support) then I think we should go with a
parameterized type solution, and I have no idea how to make it efficient.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/web-devel/attachments/20110403/f9f0a909/attachment.htm>

More information about the web-devel mailing list