```ruby
require 'alf'

class Database < Alf::Database

  helpers do

    def orders
      Relation(
        Tuple(oid: "O1", city: "London", amount: 100.00),
        Tuple(oid: "O2", city: "Paris",  amount: 100.00) 
      )
    end

    def discount_rules
      Relation(
        Tuple(city: "London", rule: ->{ amount * 0.20 }),
        Tuple(city: "Paris",  rule: ->{ amount * 0.15 })
      )
    end

    def eval(fn, scope)
      Alf::Types::TupleExpression.coerce(fn).evaluate(scope)
    end

  end

end

# Let open a connection on this database
connection = Database.connect

# Compute the discount to apply according to the city
rel = connection.query do
  allbut(
    extend( 
      join(orders, discount_rules), 
      discount: ->{ eval(rule, self) } ),
    [:rule]
  )
end

puts rel
# +------+--------+---------+-----------+
# | :oid | :city  | :amount | :discount |
# +------+--------+---------+-----------+
# | O1   | London | 100.000 |    20.000 |
# | O2   | Paris  | 100.000 |    15.000 |
# +------+--------+---------+-----------+
```

Use instance_exec instead of own hack.
user
```ruby
require 'alf'

class Database < Alf::Database

  helpers do

    def orders
      Relation(
        Tuple(oid: "O1", city: "London", amount: 100.00),
        Tuple(oid: "O2", city: "Paris",  amount: 100.00) 
      )
    end

    def discount_rules
      Relation(
        Tuple(city: "London", rule: ->{ amount * 0.20 }),
        Tuple(city: "Paris",  rule: ->{ amount * 0.15 })
      )
    end

  end

end

# Let open a connection on this database
connection = Database.connect

# Compute the discount to apply according to the city
rel = connection.query do
  allbut(
    extend( 
      join(orders, discount_rules), 
      discount: ->{ instance_exec(&rule) } ),
    [:rule]
  )
end

puts rel
# +------+--------+---------+-----------+
# | :oid | :city  | :amount | :discount |
# +------+--------+---------+-----------+
# | O1   | London | 100.000 |    20.000 |
# | O2   | Paris  | 100.000 |    15.000 |
# +------+--------+---------+-----------+
```
assistant