Setup and Testing to prevent duplication in ActiveRecord many to many relationship - activerecord

Question

I have the following classes for a many to many relationship between "Item" and "Color".
And "Item" should not have duplicated "Colors",
for example:-
If "Item1" has "Blue" & "Red" then we cannot add another "Red" to "Item1"
Is this the correct way to set this up?
class Item < ActiveRecord::Base
has_many :item_colors
has_many :colors, :through => item_colors
end
class Color < ActiveRecord::Base
has_many :item_colors
has_many :items, :through => item_colors
end
class ItemColor < ActiveRecord::Base
belongs_to :item
belongs_to :color
validates_uniqueness_of :color, scope: :item
end
My Test for duplicated colors. Is it how to test it?
describe "item should not have duplicated colors" do
before do
#item = FactoryGirl.create(:item)
#color1 = FactoryGirl.create(:color)
#item.colors << #color1
#item.colors << #color1
#item.save
end
it { should_not be_valid }
end
When I try this in rails console, it will fail when I add duplcated color to an item
but instead of getting an error message in item.errors.message, I got an ActiveRecord exception
"ActiveRecord::RecordInvalid: Validation failed: Color has already been taken"
Please advise.

Answer 1

When you add the second color, it is automatically saved because the parent object #item is already saved, i.e. it is not a new_record.
Given it is a has_many :through association, it is always saved with the bang version of save!, which in turn raises the exception because your join model ItemColor fails on the validation of uniqueness.
In your case you have two options:
rescue the exception and manage the error messages manually or;
if you're using a join model just to add the validation layer, you could get rid of it, use a HABTM instead and handle the association as a set, e.g.
> item = FactoryGirl.create(:item)
> color = FactoryGirl.create(:color)
> 10.times { item.colors |= [color] } # you can add it n times...
> item.colors.count # => 1 ...still only one is saved b/c it is a union set.
How does that sound to you?
UPDATE: In case you really want to show an error message, you could, e.g.
if item.colors.include?(color)
item.errors.add(:colors, "color already selected")
else
item.colors |= [color]
end