[elephant-devel] Re: blog example w/ hunchentoot

[Mac Chan]

Hi Ian,

On 4/21/07, Ian Eslick <eslick at csail.mit.edu> wrote:
> This example could be done even more cleanly using the class indexing
> facilities.  If you feel inclined to experiment with them, this would
> be a good contribution to improve the example.

Actually it turned out to be an interesting exercise. Please see the
attached file.

I have some questions, though.


* Displaying all the blog entries in descending order

Using the b-tree index, one could iterate through the entries with a
cursor as shown in the original example code:

(with-btree-cursor (curs *my-blog*)
  (loop
   for (m k v) = (multiple-value-list (cursor-first curs))
   then (multiple-value-list (cursor-next curs))
   while m
   collect v))

Now after I replace the explicit index with a class index, I couldn't
figure out how to use cursor on the class index. Maybe there's an
undocumented way to do this?

(loop
 for v in
 (sort (get-instances-by-class 'blog-entry) #'> :key #'entry-date)
 collect v)

Now I have to get all the instances from that class. I assumed the
result returned by get-instances-by-class is just a list of OID's?
Elephant will only fetch the values when I use the accessor functions
on the OID handle, right?

Another downside with this approach is that I need to manually sort
the entries (I guess if my assumption from the previous paragraph is
true, then I'm really paying the penalty here if there are a lot of
entries to sort) whereas in the original code it's taken care of by
the cursor API.




* Displaying all the blog entries for a particular month in descending
order

(with-btree-cursor (curs (get-index *my-blog* 'month))
  (loop
   for (m k v) = (multiple-value-list
                  (cursor-set curs month-year))
   then (multiple-value-list (cursor-next-dup curs))
   while m
   collect v))

(loop
 for v in
 (multiple-value-bind (start end)
     (the-month-range month-year)
   (nreverse (get-instances-by-range 'blog-entry 'date start end)))
 collect v)

In this case both the cursor and the get-instances-by-range API are
rather easy to use. Unlike get-instances-by-class,
get-instances-by-range seems to always return the result sorted, is
that correct?

When I first read the documenation, I couldn't think of a use case for
cursor-next-dup.  It is really handy as shown in the original example.




* Displaying href links to previous blog entries (month/year)

(loop
 for (m k v) = (multiple-value-list (cursor-first curs))
 then (multiple-value-list (cursor-next-nodup curs))
 while m
 collect v)))

(let ((month-year))
  (loop
   for entry in (get-instances-by-class 'blog-entry) do
   (multiple-value-bind (second minute hour date month year)
       (decode-universal-time (entry-date entry))
     (declare (ignore second minute hour date))
     (pushnew (encode-universal-time 0 0 0 1 month year) month-year)))
  (sort month-year #'>))

Now the cursor API is much easier to use in this case. Again, since I
can't figure out a way to use cursor on the class index, I need to go
through _all_ the entries, collect and sort the date myself.

I believe I'm not using this stuff right, so please show us the way :-)

Regards,
-- Mac
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