Extract #build_add_column_definition

Exposes AlterTable schema definition for adding new columns through new API,
and stores ddl on the schema definition. Refactors existing #add_column method to use
this as an intermediate method to build the create add_column definition.

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

@@ -26,6 +26,7 @@ def visit_AlterTable(o)
           sql << o.foreign_key_drops.map { |fk| visit_DropForeignKey fk }.join(" ")
           sql << o.check_constraint_adds.map { |con| visit_AddCheckConstraint con }.join(" ")
           sql << o.check_constraint_drops.map { |con| visit_DropCheckConstraint con }.join(" ")
+          o.ddl = sql
         end
 
         def visit_ColumnDefinition(o)

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

@@ -580,6 +580,7 @@ class AlterTable # :nodoc:
       attr_reader :adds
       attr_reader :foreign_key_adds, :foreign_key_drops
       attr_reader :check_constraint_adds, :check_constraint_drops
+      attr_accessor :ddl
 
       def initialize(td)
         @td   = td

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

@@ -599,6 +599,24 @@ def drop_table(table_name, **options)
       #  # Ignores the method call if the column exists
       #  add_column(:shapes, :triangle, 'polygon', if_not_exists: true)
       def add_column(table_name, column_name, type, **options)
+        add_column_def = build_add_column_definition(table_name, column_name, type, **options)
+        return unless add_column_def
+
+        execute(add_column_def.ddl)
+      end
+
+      def add_columns(table_name, *column_names, type:, **options) # :nodoc:
+        column_names.each do |column_name|
+          add_column(table_name, column_name, type, **options)
+        end
+      end
+
+      # Builds an AlterTable object for adding a column to a table.
+      #
+      # This definition object contains information about the column that would be created
+      # if the same arguments were passed to #add_column. See #add_column for information about
+      # passing a +table_name+, +column_name+, +type+ and other options that can be passed.
+      def build_add_column_definition(table_name, column_name, type, **options) # :nodoc:
         return if options[:if_not_exists] == true && column_exists?(table_name, column_name)
 
         if supports_datetime_with_precision?
@@ -607,15 +625,10 @@ def add_column(table_name, column_name, type, **options)
           end
         end
 
-        at = create_alter_table table_name
-        at.add_column(column_name, type, **options)
-        execute schema_creation.accept at
-      end
-
-      def add_columns(table_name, *column_names, type:, **options) # :nodoc:
-        column_names.each do |column_name|
-          add_column(table_name, column_name, type, **options)
-        end
+        alter_table = create_alter_table(table_name)
+        alter_table.add_column(column_name, type, **options)
+        schema_creation.accept(alter_table)
+        alter_table
       end
 
       # Removes the given columns from the table definition.

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

@@ -10,6 +10,7 @@ def visit_AlterTable(o)
             sql << o.constraint_validations.map { |fk| visit_ValidateConstraint fk }.join(" ")
             sql << o.exclusion_constraint_adds.map { |con| visit_AddExclusionConstraint con }.join(" ")
             sql << o.exclusion_constraint_drops.map { |con| visit_DropExclusionConstraint con }.join(" ")
+            o.ddl = sql
           end
 
           def visit_AddForeignKey(o)

activerecord/test/cases/migration/schema_definitions_test.rb

@@ -61,6 +61,23 @@ def test_build_create_index_definition_for_existing_index
           connection.drop_table(:test) if connection.table_exists?(:test)
         end
       end
+
+      def test_build_add_column_definition
+        connection.create_table(:test)
+        add_col_td = connection.build_add_column_definition(:test, :foo, :string)
+
+        assert_match "ALTER TABLE", add_col_td.ddl
+
+        add_cols = add_col_td.adds
+        assert_equal 1, add_cols.size
+
+        add_col = add_cols.first.column
+        assert_equal "foo", add_col.name
+        assert add_col.type
+        assert add_col.sql_type
+      ensure
+        connection.drop_table(:test) if connection.table_exists?(:test)
+      end
     end
   end
 end