In older migrations, allow column collation to be explicitly specified

Author: aharpole

activerecord/lib/active_record/migration/compatibility.rb

@@ -122,7 +122,7 @@ def rename_table(table_name, new_name, **options)
         def change_column(table_name, column_name, type, **options)
           options[:_skip_validate_options] = true
           if connection.adapter_name == "Mysql2" || connection.adapter_name == "Trilogy"
-            options[:collation] = :no_collation
+            options[:collation] ||= :no_collation
           end
           super
         end

activerecord/test/cases/migration/compatibility_test.rb

@@ -606,6 +606,21 @@ def migrate(x)
         connection.drop_table :more_testings rescue nil
       end
 
+      if current_adapter?(:Mysql2Adapter, :TrilogyAdapter)
+        def test_change_table_collation_not_unset_7_0
+          migration = Class.new(ActiveRecord::Migration[7.0]) {
+            def migrate(x)
+              add_column :testings, :txt_collated, :string, charset: "utf8mb4", collation: "utf8mb4_general_ci"
+              change_column :testings, :txt_collated, :string, default: "hi", collation: "utf8mb4_esperanto_ci"
+            end
+          }.new
+          ActiveRecord::Migrator.new(:up, [migration], @schema_migration, @internal_metadata).migrate
+          column = @connection.columns(:testings).find { |c| c.name == "txt_collated" }
+          assert_equal "hi", column.default
+          assert_equal "utf8mb4_esperanto_ci", column.collation
+        end
+      end
+
       def test_add_reference_on_6_0
         create_migration = Class.new(ActiveRecord::Migration[6.0]) {
           def version; 100 end