Motivation

Currently it is not possible in migrations to add a USING clause to changing the column type that have no direct available cast, without the use of writing SQL queries myself. The example that I happened to come across myself is changing the type of a column from bigint to bigint[] (array).

The postgres query to achieve this is as follows:

ALTER TABLE table_name ALTER column_name TYPE bigint[] USING array[column_name];

Proposed Solutions

Something like:

manager.alter_table(
    Table::alter()
    .table(type::Table)
    .modify_column(ColumnDef::new(type::ColumnName).array(Type::BigInteger, SimpleExpr::Using(type::ColumnName)))
    .to_owned());

Additional Information

I was requested to open this issue in a Discord thread titled "(PG) Change type of column to array of the previous type."