Rails3 Arel visits to custom classes

Question

I have a custom class, but I want to be able to pass it to Arel and let it resolve its queryable part.

module Custom
  class Item
    attr_accessor :name

    def initialize(name)
      self.name = name
    end
  end
end

custom_item = Custom::Item.new("Bill")
User.where(:name => custom_item)

is there anything I can define in custom_item, so it would understand that Arel wants name from it?

Currently I workaround with:

module Arel
  module Visitors
    class ToSql
      def visit_Custom_Item o
        "'#{o.name}'"
      end
    end
  end
end

Answer 1

I think it's not possible now. Let's try to figure out why.

When you try to pass your custom object to the where method User.where(:name => custom_item) Arel try to call visit_YOUR_CLASS_NAME method (https://github.com/rails/arel/blob/master/lib/arel/visitors/visitor.rb#L15) but it fails. Then Arel try to call visit_+object.class.ancestors.find {|klass| respond_to?(DISPATCH[klass], true)} but it fails again because:

irb(main):001:0> class Foo
irb(main):002:1> end
=> nil
irb(main):003:0> foo = Foo.new
=> #<Foo:0x261edc0>
irb(main):004:0> foo.class.ancestors.map {|klass| p klass}
Foo
Object
Kernel
BasicObject
=> [Foo, Object, Kernel, BasicObject]

and Arel doesn't have any of this names in this list https://github.com/rails/arel/blob/master/lib/arel/visitors/to_sql.rb#L394:

alias :visit_ActiveSupport_Multibyte_Chars :quoted
alias :visit_ActiveSupport_StringInquirer  :quoted
alias :visit_BigDecimal                    :quoted
alias :visit_Class                         :quoted
alias :visit_Date                          :quoted
alias :visit_DateTime                      :quoted
alias :visit_FalseClass                    :quoted
alias :visit_Float                         :quoted
alias :visit_Hash                          :quoted
alias :visit_NilClass                      :quoted
alias :visit_String                        :quoted
alias :visit_Symbol                        :quoted
alias :visit_Time                          :quoted
alias :visit_TrueClass                     :quoted

Actually, I don't think it's a very useful feature but I've asked Aaron Patterson about this and if he'll like it I'll try to implement it.

Answer 2

I was able to circumvent this problem by defining to_s method for my class

class CustomItem
  def to_s
    custom_id
  end
end

And tricking Arel Visitor to treat my class as Fixnum

Arel::Visitors::Visitor::DISPATCH[CustomItem] = 'visit_Fixnum'

Worked with Rails 3.2.13.

For string-like value you should probably mimick String and use visit_String.