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 ActiveRecord::IrreversibleMigration error.

class IrreversibleMigrationExample < ActiveRecord::Migration[5.0]
  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[5.0]
  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[5.0]
  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