Fix backfill_column_for_type_change_in_background for cast expressions

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## master (unreleased)
 
+- Fix `backfill_column_for_type_change_in_background` for cast expressions
 - Fix copying indexes with long names when changing column type
 - Enhance error messages with the link to the detailed description
 

lib/online_migrations/background_migrations/copy_column.rb

@@ -42,12 +42,18 @@ def process_batch(relation)
         old_values = copy_from.map do |from_column|
           old_value = arel_table[from_column]
           if (type_cast_function = type_cast_functions[from_column])
-            if Utils.ar_version <= 5.2
-              # Active Record <= 5.2 does not support quoting of Arel::Nodes::NamedFunction
-              old_value = Arel.sql("#{type_cast_function}(#{connection.quote_column_name(from_column)})")
-            else
-              old_value = Arel::Nodes::NamedFunction.new(type_cast_function, [old_value])
-            end
+            old_value =
+              if type_cast_function =~ /\A\w+\z/
+                if Utils.ar_version <= 5.2
+                  # Active Record <= 5.2 does not support quoting of Arel::Nodes::NamedFunction
+                  Arel.sql("#{type_cast_function}(#{connection.quote_column_name(from_column)})")
+                else
+                  Arel::Nodes::NamedFunction.new(type_cast_function, [old_value])
+                end
+              else
+                # We got a cast expression.
+                Arel.sql(type_cast_function)
+              end
           end
           old_value
         end

test/background_migrations/copy_column_test.rb

@@ -52,6 +52,14 @@ def test_process_batch_type_cast_function
       assert_equal "value", @project1.settings_for_type_change["key"]
     end
 
+    def test_process_batch_type_cast_expression
+      m = OnlineMigrations::BackgroundMigrations::CopyColumn.new(:projects, ["settings"], ["settings_for_type_change"], nil, { "settings" => "CAST(settings AS jsonb)" })
+      m.process_batch(m.relation)
+
+      @project1.reload
+      assert_equal "value", @project1.settings_for_type_change["key"]
+    end
+
     def test_count
       m = OnlineMigrations::BackgroundMigrations::CopyColumn.new(:projects, ["id"], ["id_for_type_change"])
       assert_kind_of Integer, m.count