[SOLVED] Rails PostgreSQL check if column is virtual

Rails PostgreSQL check if column is virtual

PostgreSQL 12 has a cool Generated Columns feature but I can't check whether column is virtual or not in Rails.

For reference, MySQL Adapter in Rails allows you to do like

MyModel.supports_virtual_columns?
=> true
MyModel.columns.last.virtual?
=> false

But in PostgreSQL Adapter you can't do the same despite it does support Generated fields?

Rails v.5.1.7

Solution

The fact that PostgreSQL 12 implements generated columns doesn't mean that you're going to have the interface through the Rails PostgreSQLAdapter to check whether a column is virtual or not because it's not implemented (yet).

If you see in the ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter the code for supports_virtual_columns? is pretty simple:

def supports_virtual_columns?
  mariadb? || database_version >= "5.7.5"
end

While the code to check if a column is virtual or not is stated as:

def virtual?
  /\b(?:VIRTUAL|STORED|PERSISTENT)\b/.match?(extra)
end

Which checks if the Extra value returned from a

SHOW FULL FIELDS FROM table_name;

query for that column matches with VIRTUAL, STORED or PERSISTENT, if so, it returns true.

But this doesn't exist in the PostgreSQL counterpart. supports_virtual_columns? will return false in any adapter where this feature isn't developed, as it's defined in the ActiveRecord::ConnectionAdapters::AbstractAdapter to return false:

# Does this adapter support virtual columns?
def supports_virtual_columns?
  false
end