Use another column as default value in migration

I am trying to add a new column to existing tableName table which has a column anotherColumn.

exports.up = function (knex, Promise) {
    return knex.schema.table('tableName', table => {
        table.string('newColumn').defaultTo(table.anotherColumn);
    });
};

How should I get the value of the existing column into this new column?

Answer

The short answer is, you can’t: the defaultTo value can’t come from another column. However, if you’re just trying to have the default take place at the time of migration you could do this:

exports.up = knex =>
  knex.schema.table('tableName', t => {
    t.string('newColumn')
  })
    .then(() => knex('tableName').update('newColumn', knex.ref('anotherColumn'));

It should hopefully be obvious that this will not update new rows being inserted following the migration: for that you’d need a trigger, or to ensure that you covered it in your insert code.