Exception: ActiveRecord::IrreversibleMigration

::Exception that can be raised to stop migrations from being rolled back. For example the following migration is not reversible. Rolling back this migration will raise an IrreversibleMigration error.

class IrreversibleMigrationExample < ActiveRecord::Migration[6.0]
  def change
    create_table :distributors do |t|
      t.string :zipcode
    end

    execute <<~SQL
      ALTER TABLE distributors
        ADD CONSTRAINT zipchk
          CHECK (char_length(zipcode) = 5) NO INHERIT;
    SQL
  end
end

There are two ways to mitigate this problem.

1. Define #up and #down methods instead of #change:

class ReversibleMigrationExample < ActiveRecord::Migration[6.0]
  def up
    create_table :distributors do |t|
      t.string :zipcode
    end

    execute <<~SQL
      ALTER TABLE distributors
        ADD CONSTRAINT zipchk
          CHECK (char_length(zipcode) = 5) NO INHERIT;
    SQL
  end

  def down
    execute <<~SQL
      ALTER TABLE distributors
        DROP CONSTRAINT zipchk
    SQL

    drop_table :distributors
  end
end

1. Use the #reversible method in #change method:

class ReversibleMigrationExample < ActiveRecord::Migration[6.0]
  def change
    create_table :distributors do |t|
      t.string :zipcode
    end

    reversible do |dir|
      dir.up do
        execute <<~SQL
          ALTER TABLE distributors
            ADD CONSTRAINT zipchk
              CHECK (char_length(zipcode) = 5) NO INHERIT;
        SQL
      end

      dir.down do
        execute <<~SQL
          ALTER TABLE distributors
            DROP CONSTRAINT zipchk
        SQL
      end
    end
  end
end