push alter table add column sql in to the schema modification visitor

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

@@ -262,12 +262,7 @@ def references(*args)
       end
       alias :belongs_to :references
 
-      private
-      def create_column_definition(name, type)
-        ColumnDefinition.new name, type
-      end
-
-      def new_column_definition(name, type, options)
+      def new_column_definition(name, type, options) # :nodoc:
         column = create_column_definition name, type
         limit = options.fetch(:limit) do
           native[type][:limit] if native[type].is_a?(Hash)
@@ -281,6 +276,11 @@ def new_column_definition(name, type, options)
         column
       end
 
+      private
+      def create_column_definition(name, type)
+        ColumnDefinition.new name, type
+      end
+
       def primary_key_column_name
         primary_key_column = columns.detect { |c| c.primary_key? }
         primary_key_column && primary_key_column.name
@@ -291,6 +291,23 @@ def native
       end
     end
 
+    class AlterTable # :nodoc:
+      attr_reader :add
+
+      def initialize(td)
+        @td  = td
+        @add = nil
+      end
+
+      def name; @td.name; end
+
+      def add_column(name, type, options)
+        name = name.to_s
+        type = type.to_sym
+        @add = @td.new_column_definition(name, type, options)
+      end
+    end
+
     # Represents an SQL table in an abstract way for updating a table.
     # Also see TableDefinition and SchemaStatements#create_table
     #

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

@@ -355,9 +355,9 @@ def drop_table(table_name, options = {})
       # Adds a new column to the named table.
       # See TableDefinition#column for details of the options you can use.
       def add_column(table_name, column_name, type, options = {})
-        add_column_sql = "ALTER TABLE #{quote_table_name(table_name)} ADD #{quote_column_name(column_name)} #{type_to_sql(type, options[:limit], options[:precision], options[:scale])}"
-        add_column_options!(add_column_sql, options)
-        execute(add_column_sql)
+        at = create_alter_table table_name
+        at.add_column(column_name, type, options)
+        execute schema_creation.accept at
       end
 
       # Removes the given columns from the table definition.
@@ -829,6 +829,10 @@ def create_table_definition(name, temporary, options)
         TableDefinition.new native_database_types, name, temporary, options
       end
 
+      def create_alter_table(name)
+        AlterTable.new create_table_definition(name, false, {})
+      end
+
       def update_table_definition(table_name, base)
         Table.new(table_name, base)
       end

activerecord/lib/active_record/connection_adapters/abstract_adapter.rb

@@ -113,14 +113,22 @@ def accept(o)
 
         private
 
+        def visit_AlterTable(o)
+          sql = "ALTER TABLE #{quote_table_name(o.name)} "
+
+          if col = o.add
+            sql_type = type_to_sql(col.type.to_sym, col.limit, col.precision, col.scale)
+            sql << "ADD #{quote_column_name(col.name)} #{sql_type}"
+            add_column_options!(sql, column_options(col))
+          end
+
+          sql
+        end
+
         def visit_ColumnDefinition(o)
           sql_type = type_to_sql(o.type.to_sym, o.limit, o.precision, o.scale)
           column_sql = "#{quote_column_name(o.name)} #{sql_type}"
-          column_options = {}
-          column_options[:null] = o.null unless o.null.nil?
-          column_options[:default] = o.default unless o.default.nil?
-          column_options[:column] = o
-          add_column_options!(column_sql, column_options) unless o.primary_key?
+          add_column_options!(column_sql, column_options(o)) unless o.primary_key?
           column_sql
         end
 
@@ -132,6 +140,14 @@ def visit_TableDefinition(o)
           create_sql
         end
 
+        def column_options(o)
+          column_options = {}
+          column_options[:null] = o.null unless o.null.nil?
+          column_options[:default] = o.default unless o.default.nil?
+          column_options[:column] = o
+          column_options
+        end
+
         def quote_column_name(name)
           @conn.quote_column_name name
         end