Use the original string if humanization results in a blank string (#27161)

This is to enable _ as field name.

modules/drivers/mongo/test/metabase/driver/mongo_test.clj

@@ -452,3 +452,20 @@
                                  [:= $list_field "value_lf_a"]]
                    :aggregation [[:count]]
                    :breakout    [$coll.metas.group_field]}))))))))
+
+;; Make sure that simple `_` columns can be queried (#4647)
+(tx/defdataset underscore-column
+  [["bird_species"
+    [{:field-name "name", :base-type :type/Text}
+     {:field-name "_", :base-type :type/Text}]
+    [["House Finch" "sunflower seeds, ants, nettle, dandelion"]
+     ["Mourning Dove" "millet seeds, breadcrumbs, ice cream"]]]])
+
+(deftest underscore-filter-test
+  (testing "Simple `_` columns should be possible to query (#4647)"
+    (mt/test-driver :mongo
+      (mt/dataset underscore-column
+        (is (= [[1 "House Finch" "sunflower seeds, ants, nettle, dandelion"]]
+               (mt/rows
+                (mt/run-mbql-query bird_species
+                  {:filter [:contains $bird_species._ "nett"]}))))))))

src/metabase/models/humanization.clj

@@ -51,9 +51,12 @@
   ([_ ^String s]
    ;; explode on hyphens, underscores, and spaces
    (when (seq s)
-     (str/join " " (for [part  (str/split s #"[-_\s]+")
-                         :when (not (str/blank? part))]
-                     (capitalize-word part))))))
+     (let [humanized (str/join " " (for [part  (str/split s #"[-_\s]+")
+                                         :when (not (str/blank? part))]
+                                     (capitalize-word part)))]
+       (if (str/blank? humanized)
+         s
+         humanized)))))
 
 ;; actual advanced method has been excised. this one just calls out to simple
 (defmethod name->human-readable-name :advanced

test/metabase/models/humanization_test.clj

@@ -9,8 +9,8 @@
 (deftest simple-humanization-test
   (doseq [[input expected] {nil                         nil
                             ""                          nil
-                            "_"                         ""
-                            "-"                         ""
+                            "_"                         "_"
+                            "-"                         "-"
                             "_id"                       "ID"
                             "uid"                       "UID"
                             "uuid"                      "UUID"