Virtual/generated column support for MySQL 5.7.5+ and MariaDB 5.2.0+

MySQL generated columns: https://dev.mysql.com/doc/refman/5.7/en/create-table-generated-columns.html
MariaDB virtual columns: https://mariadb.com/kb/en/mariadb/virtual-computed-columns/

Declare virtual columns with `t.virtual name, type: …, as: "expression"`.
Pass `stored: true` to persist the generated value (false by default).

Example:

    create_table :generated_columns do |t|
      t.string  :name
      t.virtual :upper_name, type: :string,  as: "UPPER(name)"
      t.virtual :name_length, type: :integer, as: "LENGTH(name)", stored: true
      t.index :name_length  # May be indexed, too!
    end

Closes #22589

1 files changed, 9 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_dumper.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_dumper.rb
index b912d24626..d6c912a69e 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_dumper.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_dumper.rb
@@ -7,7 +7,7 @@ module ActiveRecord
     # Adapter level by over-writing this code inside the database specific adapters
     module ColumnDumper
       def column_spec(column)
-        [schema_type(column), prepare_column_options(column)]
+        [schema_type_with_virtual(column), prepare_column_options(column)]
       end
 
       def column_spec_for_primary_key(column)
@@ -59,6 +59,14 @@ module ActiveRecord
           schema_type(column) == :bigint
         end
 
+        def schema_type_with_virtual(column)
+          if supports_virtual_columns? && column.virtual?
+            :virtual
+          else
+            schema_type(column)
+          end
+        end
+
         def schema_type(column)
           if column.bigint?
             :bigint