QBE example

gistfile1.rb

module Query
  
  class Expression
    attr_reader :field, :operator, :target
  
    def initialize(field, operator, target)
      @field, @operator, @target = field, operator, target
    end
  end

  class GreaterThanExpression < Expression
    def initialize(field, target)
      super field, :>, target
    end
  end
  
  class Field
    attr_reader :name
  
    def initialize(name)
      @name = name
    end
  
    def >(target)
      GreaterThanExpression.new(self, target)
    end
  end
  
  class User
    def age
      @age ||= Field.new(:age)
    end
  end
end

class User
  def self.where
    yield Query::User.new
  end
end

User.where { |user| user.age > 5 }
# => #<GreaterThanExpression:0x7baf88be @operator=:>, @field=#<Field:0x413ab162 @name=:age>, @target=5>