Enable query plan explanation for the aggregation framework

Add the explain-aggregate function, which returns a map containing
information about the given aggregation query.

src/clojure/monger/collection.clj

@@ -515,7 +515,16 @@
         res (.aggregate coll pipe agg-opts)]
     (map #(from-db-object % true) (iterator-seq res))))
 
+(defn explain-aggregate
+  "Returns the explain plan for an aggregation query. MongoDB 2.2+ only.
 
+  See http://docs.mongodb.org/manual/applications/aggregation/ to learn more."
+  [^DB db ^String coll stages & opts]
+  (let [coll (.getCollection db coll)
+        agg-opts (build-aggregation-options opts)
+        pipe (java.util.ArrayList. (to-db-object stages))
+        res (.explainAggregate coll pipe agg-opts)]
+    (from-db-object res true)))
 ;;
 ;; Misc
 ;;

test/monger/test/aggregation_framework_test.clj

@@ -104,4 +104,14 @@
       (is (= 6 (mc/count db coll)))
       (let [result (set (map #(select-keys % [:state :quantity])
                              (mc/aggregate db coll [{$project {:state 1 :quantity 1}}] :cursor {:batch-size 10})))]
-        (is (= expected result))))))
+        (is (= expected result)))))
+
+  (deftest test-explain-aggregate
+    (let [batch [{:state "CA" :price 100}
+                 {:state "CA" :price 10}
+                 {:state "IL" :price 50}]
+          expected-keys #{:ok :stages}]
+      (mc/insert-batch db coll batch)
+      (let [result (mc/explain-aggregate db coll [{$match {:state "CA"}}])
+            key-in-result? (partial contains? result)]
+        (is (every? key-in-result? expected-keys))))))