Add reversible syntax for change_column_default

Passing `:from` and `:to` to `change_column_default` makes this command
reversible as user has defined its previous state.

So, instead of having the migration command as:

    change_column_default(:posts, :state, "draft")

They can write it as:

    change_column_default(:posts, :state, from: nil, to: "draft")

activerecord/CHANGELOG.md

@@ -1,3 +1,15 @@
+*   Add alternate syntax to make `change_column_default` reversible.
+
+    User can pass in `:from` and `:to` to make `change_column_default` command
+    become reversible.
+
+    Example:
+
+        change_column_default :posts, :status, from: nil, to: "draft"
+        change_column_default :users, authorized, from: true, to: false
+
+    *Prem Sichanugrist*
+
 *   Prevent error when using `force_reload: true` on an unassigned polymorphic
     belongs_to association.
 

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

@@ -592,10 +592,11 @@ def change(column_name, type, options = {})
       #
       #  t.change_default(:qualification, 'new')
       #  t.change_default(:authorized, 1)
+      #  t.change_default(:status, from: nil, to: "draft")
       #
       # See SchemaStatements#change_column_default
-      def change_default(column_name, default)
-        @base.change_column_default(name, column_name, default)
+      def change_default(column_name, default_or_changes)
+        @base.change_column_default(name, column_name, default_or_changes)
       end
 
       # Removes the column(s) from the table definition.

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

@@ -460,7 +460,12 @@ def change_column(table_name, column_name, type, options = {})
       #
       #   change_column_default(:users, :email, nil)
       #
-      def change_column_default(table_name, column_name, default)
+      # Passing a hash containing +:from+ and +:to+ will make this change
+      # reversible in migration:
+      #
+      #   change_column_default(:posts, :state, from: nil, to: "draft")
+      #
+      def change_column_default(table_name, column_name, default_or_changes)
         raise NotImplementedError, "change_column_default is not implemented"
       end
 
@@ -1068,6 +1073,14 @@ def validate_index_length!(table_name, new_name) # :nodoc:
           raise ArgumentError, "Index name '#{new_name}' on table '#{table_name}' is too long; the limit is #{allowed_index_name_length} characters"
         end
       end
+
+      def extract_new_default_value(default_or_changes)
+        if default_or_changes.is_a?(Hash) && default_or_changes.has_key?(:from) && default_or_changes.has_key?(:to)
+          default_or_changes[:to]
+        else
+          default_or_changes
+        end
+      end
     end
   end
 end

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -629,7 +629,8 @@ def rename_index(table_name, old_name, new_name)
         end
       end
 
-      def change_column_default(table_name, column_name, default) #:nodoc:
+      def change_column_default(table_name, column_name, default_or_changes) #:nodoc:
+        default = extract_new_default_value(default_or_changes)
         column = column_for(table_name, column_name)
         change_column table_name, column_name, column.sql_type, :default => default
       end

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

@@ -410,11 +410,12 @@ def change_column(table_name, column_name, type, options = {}) #:nodoc:
         end
 
         # Changes the default value of a table column.
-        def change_column_default(table_name, column_name, default) # :nodoc:
+        def change_column_default(table_name, column_name, default_or_changes) # :nodoc:
           clear_cache!
           column = column_for(table_name, column_name)
           return unless column
 
+          default = extract_new_default_value(default_or_changes)
           alter_column_query = "ALTER TABLE #{quote_table_name(table_name)} ALTER COLUMN #{quote_column_name(column_name)} %s"
           if default.nil?
             # <tt>DEFAULT NULL</tt> results in the same behavior as <tt>DROP DEFAULT</tt>. However, PostgreSQL will

activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb

@@ -422,7 +422,9 @@ def remove_column(table_name, column_name, type = nil, options = {}) #:nodoc:
         end
       end
 
-      def change_column_default(table_name, column_name, default) #:nodoc:
+      def change_column_default(table_name, column_name, default_or_changes) #:nodoc:
+        default = extract_new_default_value(default_or_changes)
+
         alter_table(table_name) do |definition|
           definition[column_name].default = default
         end

activerecord/lib/active_record/migration/command_recorder.rb

@@ -72,7 +72,7 @@ def respond_to?(*args) # :nodoc:
 
       [:create_table, :create_join_table, :rename_table, :add_column, :remove_column,
         :rename_index, :rename_column, :add_index, :remove_index, :add_timestamps, :remove_timestamps,
-        :change_column_default, :add_reference, :remove_reference, :transaction,
+        :add_reference, :remove_reference, :transaction,
         :drop_join_table, :drop_table, :execute_block, :enable_extension,
         :change_column, :execute, :remove_columns, :change_column_null,
         :add_foreign_key, :remove_foreign_key
@@ -166,6 +166,16 @@ def invert_remove_index(args)
       alias :invert_add_belongs_to :invert_add_reference
       alias :invert_remove_belongs_to :invert_remove_reference
 
+      def invert_change_column_default(args)
+        table, column, options = *args
+
+        unless options && options.is_a?(Hash) && options.has_key?(:from) && options.has_key?(:to)
+          raise ActiveRecord::IrreversibleMigration, "change_column_default is only reversible if given a :from and :to option."
+        end
+
+        [:change_column_default, [table, column, from: options[:to], to: options[:from]]]
+      end
+
       def invert_change_column_null(args)
         args[2] = !args[2]
         [:change_column_null, args]

activerecord/test/cases/migration/columns_test.rb

@@ -267,6 +267,13 @@ def test_change_column_default_to_null
         assert_nil TestModel.new.first_name
       end
 
+      def test_change_column_default_with_from_and_to
+        add_column "test_models", "first_name", :string
+        connection.change_column_default "test_models", "first_name", from: nil, to: "Tester"
+
+        assert_equal "Tester", TestModel.new.first_name
+      end
+
       def test_remove_column_no_second_parameter_raises_exception
         assert_raise(ArgumentError) { connection.remove_column("funny") }
       end

activerecord/test/cases/migration/command_recorder_test.rb

@@ -169,6 +169,16 @@ def test_invert_change_column_default
         end
       end
 
+      def test_invert_change_column_default_with_from_and_to
+        change = @recorder.inverse_of :change_column_default, [:table, :column, from: "old_value", to: "new_value"]
+        assert_equal [:change_column_default, [:table, :column, from: "new_value", to: "old_value"]], change
+      end
+
+      def test_invert_change_column_default_with_from_and_to_with_boolean
+        change = @recorder.inverse_of :change_column_default, [:table, :column, from: true, to: false]
+        assert_equal [:change_column_default, [:table, :column, from: false, to: true]], change
+      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