make change_column_comment and change_table_comment invertible

We can revert migrations using `change_column_comment` or
`change_table_comment` at current master.
However, results are not what we expect: comments are remained in new
status.
This change tells previous comment to these methods in a way like
`change_column_default`.

2 files changed, 63 insertions, 0 deletions

diff --git a/activerecord/test/cases/migration/command_recorder_test.rb b/activerecord/test/cases/migration/command_recorder_test.rb
index 01f8628fc5..c9f3756b1f 100644
--- a/activerecord/test/cases/migration/command_recorder_test.rb
+++ b/activerecord/test/cases/migration/command_recorder_test.rb
@@ -182,6 +182,40 @@ module ActiveRecord
         assert_equal [:change_column_default, [:table, :column, from: false, to: true]], change
       end
 
+      if ActiveRecord::Base.connection.supports_comments?
+        def test_invert_change_column_comment
+          assert_raises(ActiveRecord::IrreversibleMigration) do
+            @recorder.inverse_of :change_column_comment, [:table, :column, "comment"]
+          end
+        end
+
+        def test_invert_change_column_comment_with_from_and_to
+          change = @recorder.inverse_of :change_column_comment, [:table, :column, from: "old_value", to: "new_value"]
+          assert_equal [:change_column_comment, [:table, :column, from: "new_value", to: "old_value"]], change
+        end
+
+        def test_invert_change_column_comment_with_from_and_to_with_nil
+          change = @recorder.inverse_of :change_column_comment, [:table, :column, from: nil, to: "new_value"]
+          assert_equal [:change_column_comment, [:table, :column, from: "new_value", to: nil]], change
+        end
+
+        def test_invert_change_table_comment
+          assert_raises(ActiveRecord::IrreversibleMigration) do
+            @recorder.inverse_of :change_column_comment, [:table, :column, "comment"]
+          end
+        end
+
+        def test_invert_change_table_comment_with_from_and_to
+          change = @recorder.inverse_of :change_table_comment, [:table, from: "old_value", to: "new_value"]
+          assert_equal [:change_table_comment, [:table, from: "new_value", to: "old_value"]], change
+        end
+
+        def test_invert_change_table_comment_with_from_and_to_with_nil
+          change = @recorder.inverse_of :change_table_comment, [:table, from: nil, to: "new_value"]
+          assert_equal [:change_table_comment, [:table, from: "new_value", to: nil]], change
+        end
+      end
+
       def test_invert_change_column_null
         add = @recorder.inverse_of :change_column_null, [:table, :column, true]
         assert_equal [:change_column_null, [:table, :column, false]], add
diff --git a/activerecord/test/cases/migration/compatibility_test.rb b/activerecord/test/cases/migration/compatibility_test.rb
index eaf88bf698..726ccf925e 100644
--- a/activerecord/test/cases/migration/compatibility_test.rb
+++ b/activerecord/test/cases/migration/compatibility_test.rb
@@ -220,6 +220,35 @@ module ActiveRecord
         end
       end
 
+      if ActiveRecord::Base.connection.supports_comments?
+        def test_change_column_comment_can_be_reverted
+          migration = Class.new(ActiveRecord::Migration[5.2]) {
+            def migrate(x)
+              revert do
+                change_column_comment(:testings, :foo, "comment")
+              end
+            end
+          }.new
+
+          ActiveRecord::Migrator.new(:up, [migration]).migrate
+          assert connection.column_exists?(:testings, :foo, comment: "comment")
+        end
+
+        def test_change_table_comment_can_be_reverted
+          migration = Class.new(ActiveRecord::Migration[5.2]) {
+            def migrate(x)
+              revert do
+                change_table_comment(:testings, "comment")
+              end
+            end
+          }.new
+
+          ActiveRecord::Migrator.new(:up, [migration]).migrate
+
+          assert_equal "comment", connection.table_comment("testings")
+        end
+      end
+
       if current_adapter?(:PostgreSQLAdapter)
         class Testing < ActiveRecord::Base
         end