Implement change_table_comment and change_column_comment for MySql Adapter

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -311,6 +311,11 @@ def bulk_change_table(table_name, operations) #:nodoc:
         execute("ALTER TABLE #{quote_table_name(table_name)} #{sqls}")
       end
 
+      def change_table_comment(table_name, comment) #:nodoc:
+        comment = "" if comment.nil?
+        execute("ALTER TABLE #{quote_table_name(table_name)} COMMENT #{quote(comment)}")
+      end
+
       # Renames a table.
       #
       # Example:
@@ -365,6 +370,11 @@ def change_column_null(table_name, column_name, null, default = nil) #:nodoc:
         change_column table_name, column_name, column.sql_type, null: null
       end
 
+      def change_column_comment(table_name, column_name, comment) #:nodoc:
+        column = column_for(table_name, column_name)
+        change_column table_name, column_name, column.sql_type, comment: comment
+      end
+
       def change_column(table_name, column_name, type, options = {}) #:nodoc:
         execute("ALTER TABLE #{quote_table_name(table_name)} #{change_column_sql(table_name, column_name, type, options)}")
       end

activerecord/test/cases/comment_test.rb

@@ -142,5 +142,27 @@ def test_schema_dump_omits_blank_comments
       assert_match %r[t\.string\s+"absent_comment"\n], output
       assert_no_match %r[t\.string\s+"absent_comment", comment:\n], output
     end
+
+    def test_change_table_comment
+      @connection.change_table_comment :commenteds, "Edited table comment"
+      assert_equal "Edited table comment", @connection.table_comment("commenteds")
+    end
+
+    def test_change_table_comment_to_nil
+      @connection.change_table_comment :commenteds, nil
+      assert @connection.table_comment("commenteds").blank?
+    end
+
+    def test_change_column_comment
+      @connection.change_column_comment :commenteds, :name, "Edited column comment"
+      column = Commented.columns_hash["name"]
+      assert_equal "Edited column comment", column.comment
+    end
+
+    def test_change_column_comment_to_nil
+      @connection.change_column_comment :commenteds, :name, nil
+      column = Commented.columns_hash["name"]
+      assert_nil column.comment
+    end
   end
 end