Make ActiveRecord style queries using block syntax

r.rb

class Query
  attr_accessor :mappings
  attr_accessor :filters

  def initialize
    @mappings = []
    @filters = []
  end

  class ConditionProxy
    def initialize(q)
      @q = q
    end

    def method_missing(name, *args, &block)
      @name = name
      self
    end

    def <(thing)
      @q.filters << "#{@name} < #{thing}"
      @q
    end
  end

  class MapProxy
    def initialize(q)
      @q = q
    end

    def method_missing(name, *args, &block)
      @q.mappings << name
      @q
    end
  end

  def where(&block)
    block.call(ConditionProxy.new(self))
  end

  def map(&block)
    block.call(MapProxy.new(self))
  end

  def to_sql
    "SELECT #{@mappings.join(', ')} FROM table WHERE #{filters.join(' AND ')};"
  end
end

puts '=' * 80

q = Query.new
q.where { |p| p.amount < 32 }.map(&:user_id)

p q
p q.to_sql
##<Query:0x00005622190d6560 @mappings=[:user_id], @filters=["amount < 32"]>
#"SELECT user_id FROM table WHERE amount < 32;"