My answer is "Don't do that". Values in documents shouldn't depend on
values in other documents, that's a better fit for a relational or OO
DB. In your example though, CouchDB's views could be used to compute
the sums.
-Damien
On Nov 13, 2008, at 12:01 PM, ara howard wrote:
what are people's strategies for dealing with the following scenario
doc_a = get 'id_a'
doc_b = get 'id_b'
obj_c = { 'sum' : doc_a.x + doc_b.y }
put obj_c
this kind of thing is tricky even in a traditional RDBMS, since the
default transaction level may or may not allow the application to
see an uncommitted write by another transaction.
the only way i can think of to get consistency from an op like the
above would be to do
bulk_put [ obj_c, doc_a, doc_b ]
in other words, if you are ever going to compute values to from
couch docs to produce another doc, it would seem that's it's
required to put *all* read information back in order to ensure that
the sources have not changed since the time that you read them. the
issue with this, of course, is that a result computed from many
documents is going to cause exponential slowdown since the potential
for overlapping writes will increase with the number of documents
and also the size of updates themselves will increase similarly.
a solution i can image is something like
list = get 'some_view'
obj = computed_value_from list
obj[ '_depends_on' ] = list.map{|element| [element.id, element.rev]}
put obj
so basically a method to do a put with not only your rev, but that
of 'n' dependent docs where only the [id, rev] pair for the
dependent docs need be posted. am i making any sense here?
cheers.
a @ http://codeforpeople.com/
--
we can deny everything, except that we have the possibility of being
better. simply reflect on that.
h.h. the 14th dalai lama
