Why is my define_method not working? - ruby-on-rails

Question

I have two methods in my Designer class (in my rails app):
def add_specialty(specialty)
specialty_list.add(specialty)
save
end
def add_qualification(qualification)
qualification_list.add(qualification)
save
end
Here are specs I have for them that are passing:
context 'adding specialties' do
it "can add a new specialty" do
expect { designer.add_specialty("interior design") }.to change {designer.specialty_list.count}.by(1)
expect(designer.specialty_list).to include("interior design")
end
end
context 'adding qualifications' do
it "can add a new qualification" do
expect { designer.add_qualification("architect") }.to change {designer.qualification_list.count}.by(1)
expect(designer.qualification_list).to include("architect")
end
end
Now I want to refactor to this implementation:
["specialty", "qualification"].each do |attr|
define_method("add_#{attr}") do |arg|
"#{attr}_list".add(arg)
save
end
end
This fails. I get failures:
1) Designer adding qualifications can add a new qualification
Failure/Error: expect { designer.add_qualification("architect") }.to change {designer.qualification_list.count}.by(1)
NoMethodError:
undefined method `add' for "qualification_list":String
# ./app/models/designer.rb:93:in `block (2 levels) in <class:Designer>'
# ./spec/models/designer_spec.rb:79:in `block (4 levels) in <top (required)>'
# ./spec/models/designer_spec.rb:79:in `block (3 levels) in <top (required)>'
# -e:1:in `<main>'
2) Designer adding specialties can add a new specialty
Failure/Error: expect { designer.add_specialty("interior design") }.to change {designer.specialty_list.count}.by(1)
NoMethodError:
undefined method `add' for "specialty_list":String
# ./app/models/designer.rb:93:in `block (2 levels) in <class:Designer>'
# ./spec/models/designer_spec.rb:72:in `block (4 levels) in <top (required)>'
# ./spec/models/designer_spec.rb:72:in `block (3 levels) in <top (required)>'
# -e:1:in `<main>'
What am I doing wrong in my define_method implementation?

Answer 1

"#{attr}_list" by itself is just the string "specialty_list" or "qualification_list", and strings don't have an add method. I think you want to the send the specialty_list method e.g.
%w{ specialty qualification }.each do |attr|
define_method("add_#{attr}") do |arg|
send("#{attr}_list").add(arg)
save
end
end

Answer 2

Ok: I got it working like this:
["specialty", "qualification"].each do |attr|
define_method("add_#{attr}") do |arg|
instance_eval("#{attr}_list").send(:add, arg)
save
end
end
Not sure why this worked though or if its the right way to do it. Would anyone care to contribute towards a better understanding?