Add more descriptive error message when uri has no db name

src/clojure/monger/core.clj

@@ -235,8 +235,11 @@
   [^String uri-string]
   (let [uri    (MongoClientURI. uri-string)
         conn   (MongoClient. uri)
-        db   (.getDB conn (.getDatabase uri))]
-    {:conn conn :db db}))
+        dbName (.getDatabase uri)]
+    (if (nil? dbName)
+      (throw (Exception. "No database name specified in uri"))
+      (let [db (.getDB conn dbName)]
+        {:conn conn :db db}))))
 
 (defn ^com.mongodb.CommandResult command
   "Runs a database command (please check MongoDB documentation for the complete list of commands).

test/monger/test/core_test.clj

@@ -70,3 +70,7 @@
               :cursor-finalizer-enabled true
               :required-replica-set-name "rs"}]
     (is (instance? com.mongodb.MongoClientOptions$Builder (mg/mongo-options-builder opts)))))
+
+(deftest connect-to-uri-without-db-name
+  (let [uri "mongodb://localhost:27017"]
+    (is (thrown? Exception (mg/connect-via-uri uri)))))