add a function for finding the first session created for a given user (including unit test).

src/metabase/models/session.clj

 (ns metabase.models.session
-  (:require [korma.core :refer :all, :exclude [defentity update]]
-            [metabase.db :refer :all]
+  (:require [korma.core :as k]
             (metabase.models [common :refer :all]
                              [interface :refer :all]
                              [user :refer [User]])
             [metabase.util :as u]))
 
 (defentity Session
-  [(table :core_session)
-   (belongs-to User {:fk :user_id})]
+  [(k/table :core_session)
+   (k/belongs-to User {:fk :user_id})]
 
   (pre-insert [_ session]
     (let [defaults {:created_at (u/new-sql-timestamp)}]
       (merge defaults session))))
+
+
+;; Persistence Functions
+
+(defn first-session-for-user
+  "Retrieves the first Session `:id` for a given user (if available), or nil otherwise."
+  [user-id]
+  {:pre [(integer? user-id)]}
+  (k/select Session
+    (k/fields :id)
+    (k/where {:user_id user-id})
+    (k/order :created_at :ASC)
+    (k/limit 1)))

test/metabase/models/session_test.clj

+(ns metabase.models.session-test
+  (:require [expectations :refer :all]
+            [korma.core :as k]
+            [metabase.models.session :refer :all]
+            [metabase.test.data.users :refer :all]))
+
+;; first-session-for-user
+(expect
+  "the-greatest-day-ever"
+  (do
+    (k/insert Session
+              (k/values [{:id         "the-greatest-day-ever"
+                          :user_id    (user->id :rasta)
+                          :created_at (metabase.util/->Timestamp "1980-10-19")}
+                         {:id         "the-world-of-bi-changes-forever"
+                          :user_id    (user->id :rasta)
+                          :created_at (metabase.util/->Timestamp "2015-10-21")}
+                         {:id         "something-could-have-happened"
+                          :user_id    (user->id :rasta)
+                          :created_at (metabase.util/->Timestamp "1999-12-31")}
+                         {:id         "now"
+                          :user_id    (user->id :rasta)
+                          :created_at (metabase.util/new-sql-timestamp)}]))
+    (first-session-for-user (user->id :rasta))))