Aliasing Column with Hashtag in Name in Rails

Question

I have a legacy database with a column named similar to My#Column which I am trying to alias. In my Sequel model I have:

alias_attribute :MyColumn, :"My#Column"

But I get a syntax error:

...Ruby24-x64/lib/ruby/gems/2.4.0/gems/activesupport-5.1.4/lib/active_support/core_ext/module/aliasing.rb:26: syntax error, unexpected end-of-input, expecting keyword_end

The problem seems to be the #. I have tried escaping it like \# but I get the same error. I don't understand why I get a syntax error as this kind of symbol has worked for me in other places.

What do I do to get this alias to work?

Answer 1

This is what alias_attribute is actually doing Source

  module_eval <<-STR, __FILE__, __LINE__ + 1
    def #{new_name}; self.#{old_name}; end          # def subject; self.title; end
    def #{new_name}?; self.#{old_name}?; end        # def subject?; self.title?; end
    def #{new_name}=(v); self.#{old_name} = v; end  # def subject=(v); self.title = v; end
  STR

So essentially this is becoming

  def MyColumn; self.My#Column; end
  def MyColumn?; self.My#Column?; end
  def MyColumn=(val); self.My#Column= val; end

Notice the fact that this is in a single line which means that everything after self.My becomes a comment (including the end) and thus the error you are receiving. Even if this was not a single line in rails ruby would simply raise a NoMethodError for the fact that My is not a method because the #Column portion would be treated as a comment.

This also seems strange since ActiveModel#alias_attribute has a functional implementation of the exact same via define_proxy_call which looks like

def define_proxy_call(include_private, mod, name, send, *extra)
  defn = if NAME_COMPILABLE_REGEXP.match?(name)
     "def #{name}(*args)"
  else
     "define_method(:'#{name}') do |*args|"
  end

  extra = (extra.map!(&:inspect) << "*args").join(", ".freeze)

  target = if CALL_COMPILABLE_REGEXP.match?(send)
    "#{"self." unless include_private}#{send}(#{extra})"
  else
    "send(:'#{send}', #{extra})"
  end

  mod.module_eval <<-RUBY, __FILE__, __LINE__ + 1
    #{defn}
      #{target}
    end
    RUBY
end

Here you can see it actually checks to see if the new name(name) and original name(send) are "compilable" and if not it handles them appropriately.

Rather than alias_attribute which is going to have issues with what is essentially a comment character. I would suggest manually implementing the same using public_send e.g.

  def MyColumn
    self.public_send("My#Column") 
  end 
  def MyColumn=(val)
    self.public_send("My#Column=",val)
  end
  def MyColumn?
    self.public_send("My#Column?")
  end

This should result in the same but without the syntax issues.