middleware.clj

(ns foo.core
  (:require [ring.adapter.jetty :as jetty]))

(defn add-connection-middleware [handler uri]
  (fn [req]
    (let [connection (mg/connect-via-uri uri)]
      (handler (assoc req :connection connection)))))

(defn handler [{:key [connection] :as request}]
  {:status 200
   :headers {"Content-Type" "text/html"}
   :body (str "Hello World" connection)})

(jetty/run-jetty
 (add-connection-middleware handler (config-keys :mongoURI))
 {:port 3000})