Hi Andrew,
On Tuesday, January 15, 2013 at 10:23 AM, Andrew Xue wrote:
> Hi --
>
> Building some generic joining functionality on top of cascalog, the clojure
> package for big data.
>
> I have a join function that is defined like this:
>
> (defn join [lhs rhs join-ons] ..implementation ...)
>
> lhs and rhs are subqueries. the join returns then another query that is a
> join of the two.
>
> join-ons in this case would look something like this [{lhs: "user_id_lhs"
> :rhs "user_id_rhs" :as "user_id"}]
>
> each map in the vector corresponds to a join condition (in sql, the above
> would be like lhs join rhs on lhs.user_id_lhs=rhs.user_id_rhs) .. the :as
> keyword renames the join variable in the result of the join. join-ons is a
> vector of these join conditions.
>
> this works ok and its basically modeled after joins in sql.
>
> however it would be nice to able to do something maybe like this
>
> (reduce join [query1 query2 query3 ... queryN])
>
> i am having trouble picturing how the join-ons would work in this case though
> ...
The simplest way to do this is to define a function generator:
(defn generate-join-fn [join-ons]
(fn [lhs rhs]
(let [join-on-cond (join-ons [lhs rhs])]
(join lhs rhs join-on-cond))))
This will then be used as such:
(reduce
(generate-join-fn {[my-left my-right] {:lhs "myleft" :rhs "myright"}
[my-right my-final] {:lhs "myright" :rhs "myfinal"}})
[my-left my-right my-final])
Having the function that returns the actual reducer function is what is key.
HTH,
--
JM Ibanez
[email protected]
http://jmibanez.com/
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