Fix bug in field-query, add comment about cause

src/metabase/db/metadata_queries.clj

@@ -22,7 +22,9 @@
 (defn- field-query [{table-id :table_id} query]
   {:pre [(integer? table-id)]}
   (qp-query (db/select-one-field :db_id Table, :id table-id)
-            (ql/query query (ql/source-table table-id))))
+            ;; this seeming useless `merge` statement IS in fact doing something important. `ql/query` is a threading macro for building queries. Do not remove
+            (ql/query (merge query)
+                      (ql/source-table table-id))))
 
 (defn table-row-count
   "Fetch the row count of TABLE via the query processor."