Merge pull request #23345 from yui-knk/warning_when_composite_primary_key_is_detected

Warn if `AR.primary_key` is called for a table who has composite prim…

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -125,6 +125,12 @@ def column_exists?(table_name, column_name, type = nil, options = {})
       # Returns just a table's primary key
       def primary_key(table_name)
         pks = primary_keys(table_name)
+        warn <<-WARNING.strip_heredoc if pks.count > 1
+          WARNING: Rails does not support composite primary key.
+
+          #{table_name} has composite primary key. Composite primary key is ignored.
+        WARNING
+
         pks.first if pks.one?
       end
 

activerecord/test/cases/primary_keys_test.rb

@@ -260,6 +260,13 @@ def test_composite_primary_key
     assert_equal ["region", "code"], @connection.primary_keys("barcodes")
   end
 
+  def test_primary_key_issues_warning
+    warning = capture(:stderr) do
+      @connection.primary_key("barcodes")
+    end
+    assert_match(/WARNING: Rails does not support composite primary key\./, warning)
+  end
+
   def test_collectly_dump_composite_primary_key
     schema = dump_table_schema "barcodes"
     assert_match %r{create_table "barcodes", primary_key: \["region", "code"\]}, schema