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, 10 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb b/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb
index d808b50332..39c2acbca9 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb
@@ -2,8 +2,8 @@ module ActiveRecord
   module ConnectionAdapters
     module MySQL
       class SchemaCreation < AbstractAdapter::SchemaCreation
-        delegate :add_sql_comment!, to: :@conn
-        private :add_sql_comment!
+        delegate :add_sql_comment!, :mariadb?, to: :@conn
+        private :add_sql_comment!, :mariadb?
 
         private
 
@@ -32,6 +32,7 @@ module ActiveRecord
           def column_options(o)
             column_options = super
             column_options[:charset] = o.charset
+            column_options[:stored] = o.stored
             column_options
           end
 
@@ -44,6 +45,13 @@ module ActiveRecord
               sql << " COLLATE #{collation}"
             end
 
+            if as = options[:as]
+              sql << " AS (#{as})"
+              if options[:stored]
+                sql << (mariadb? ? " PERSISTENT" : " STORED")
+              end
+            end
+
             add_sql_comment!(super, options[:comment])
           end