Configure foreign key support for Mongo to allow implicit joins (#29566)

* Configure foreign key support for Mongo to allow implicit joins

modules/drivers/mongo/src/metabase/driver/mongo.clj

@@ -261,6 +261,14 @@
                  :standard-deviation-aggregations]]
   (defmethod driver/supports? [:mongo feature] [_driver _feature] true))
 
+;; We say Mongo supports foreign keys so that the front end can use implicit
+;; joins. In reality, Mongo doesn't support foreign keys.
+;; Only define an implementation for `:foreign-keys` if none exists already.
+;; In test extensions we define an alternate implementation, and we don't want
+;; to stomp over that if it was loaded already.
+(when-not (get (methods driver/supports?) [:mongo :foreign-keys])
+  (defmethod driver/supports? [:mongo :foreign-keys] [_ _] true))
+
 (defmethod driver/database-supports? [:mongo :expressions]
   [_driver _feature db]
   (-> (:dbms_version db)

modules/drivers/mongo/test/metabase/test/data/mongo.clj

@@ -6,6 +6,8 @@
    [clojure.test :refer :all]
    [flatland.ordered.map :as ordered-map]
    [medley.core :as m]
+   [metabase.config :as config]
+   [metabase.driver :as driver]
    [metabase.driver.ddl.interface :as ddl.i]
    [metabase.driver.mongo.util :refer [with-mongo-connection]]
    [metabase.test.data.interface :as tx]
@@ -21,6 +23,9 @@
 (defmethod tx/supports-time-type? :mongo [_driver] false)
 (defmethod tx/supports-timestamptz-type? :mongo [_driver] false)
 
+;; during unit tests don't treat Mongo as having FK support
+(defmethod driver/supports? [:mongo :foreign-keys] [_ _] (not config/is-test?))
+
 (defn ssl-required?
   "Returns if the mongo server requires an SSL connection."
   []
@@ -53,7 +58,7 @@
                    {:pass password}))))
 
 (defn- destroy-db! [driver dbdef]
-  (with-mongo-connection [mongo-connection (tx/dbdef->connection-details driver :server dbdef)]
+  (with-mongo-connection [^com.mongodb.DB mongo-connection (tx/dbdef->connection-details driver :server dbdef)]
     (mg/drop-db (.getMongo mongo-connection) (tx/escaped-database-name dbdef))))
 
 (def ^:dynamic *remove-nil?*