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")

5 files changed, 24 insertions, 6 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
index 158b773e11..d17e272ed1 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
@@ -592,10 +592,11 @@ module ActiveRecord
       #
       #  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.
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index 49ffd7ccf0..8bbd96e722 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -460,7 +460,12 @@ module ActiveRecord
       #
       #   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 @@ module ActiveRecord
           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
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index 00e3d2965b..2027492f29 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -629,7 +629,8 @@ module ActiveRecord
         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
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index 595c635fc0..d114cad16b 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -410,11 +410,12 @@ module ActiveRecord
         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
diff --git a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
index 87129c42cf..7c809b088c 100644
--- a/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb
@@ -422,7 +422,9 @@ module ActiveRecord
         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