[MBQL lib] Match implicitly joined refs with multiple paths (#49446)

The ref (usually) contains enough information to unambiguously find the
matching implictly-joined column, even if there are several FKs that
could join the target table.

The remaining ambiguous case is if the *same* FK column exists
multiple times - current MBQL uses raw field IDs for the FK field, so it
can't distinguish between eg. an FK in the main table and the one from a
self-join.

src/metabase/lib/equality.cljc

@@ -128,13 +128,11 @@
 (mu/defn- matching-join? :- :boolean
   [[_ref-kind {:keys [join-alias source-field]} _ref-id] :- ::lib.schema.ref/ref
    column                                                :- ::lib.schema.metadata/column]
-  ;; If the ref has a source-field, and it matches the column's :fk-field-id then this is an implicitly joined field.
-  ;; Implicitly joined columns have :source-alias ("PRODUCTS__via__PRODUCT_ID") but the refs don't have any join alias.
-  (or (and source-field
-           (clojure.core/= source-field (:fk-field-id column)))
-      ;; If it's not an implicit join, then either the join aliases must match for an explicit join, or both be nil for
-      ;; an own column.
-      (clojure.core/= (column-join-alias column) join-alias)))
+  (if source-field
+    (clojure.core/= source-field (:fk-field-id column))
+    ;; If it's not an implicit join, then either the join aliases must match for an explicit join, or both be nil for
+    ;; an own column.
+    (clojure.core/= (column-join-alias column) join-alias)))
 
 (mu/defn- plausible-matches-for-name :- [:sequential ::lib.schema.metadata/column]
   [[_ref-kind opts ref-name :as a-ref] :- ::lib.schema.ref/ref

test/metabase/lib/equality_test.cljc

@@ -677,3 +677,16 @@
               (lib.equality/find-matching-ref (first columns) column-refs)))
       (is (=? [:field {} "CREATED_AT_2"]
               (lib.equality/find-matching-ref (second columns) column-refs))))))
+
+(deftest ^:parallel find-matching-column-multiple-implicit-joins-test
+  (testing "when there are multiple implicit joins for the same column, matches correctly"
+    (let [base      (lib/query meta/metadata-provider (meta/table-metadata :ic/reports))
+          columns   (lib.metadata.calculation/visible-columns base)
+          by-id     (group-by :id columns)
+          name-cols (by-id (meta/id :ic/accounts :name))]
+      (is (= 2 (count name-cols)))
+      (doseq [col name-cols
+              :let [query (lib/filter base (lib/= col "foo"))
+                    cols  (lib.metadata.calculation/visible-columns query)
+                    [_op _opts filter-col] (first (lib/filters query))]]
+        (is (=? col (lib.equality/find-matching-column query -1 filter-col cols)))))))