```ruby
module Arel
  module Nodes
    class Extract < Arel::Nodes::Unary
      include Arel::AliasPredication
      include Arel::Predications

      attr_accessor :field

      def initialize expr, field
        super(expr)
        @field = field
      end

      def hash
        super ^ [@field, @alias].hash
      end

      def eql? other
        super &&
          self.field == other.field
      end
      alias :== :eql?
    end
  end
end
```

Remove unused @alias, being referenced in hashing.
user
```ruby
module Arel
  module Nodes
    class Extract < Arel::Nodes::Unary
      include Arel::AliasPredication
      include Arel::Predications

      attr_accessor :field

      def initialize expr, field
        super(expr)
        @field = field
      end

      def hash
        super ^ @field.hash
      end

      def eql? other
        super &&
          self.field == other.field
      end
      alias :== :eql?
    end
  end
end
```
assistant