Let's say you have a Container model, and that container can have many Parts.
There are many types of Parts (Sprockets, Springs, and Spindles), so we need some polymorphism.
- Containers have_many PartAssignments.
- PartAssignments belong_to a (polymorphic) Part.
- Part is an abstract class, which is used by Sprocket, Spindle, and Spring.
To add a Sprocket to a Container, you'd do something like this:
c = Container.create(:name => 'Super Container')
s = Sprocket.new(:name => 'Fancy Sprocket')
c.part_assignments.create(:part => s)
To add a Spring to the same container, you'd do:
spring = Spring.new(:name => 'Fancy Spring')
c.part_assignments.create(:part => spring)
At this point, you can get a list of all the parts:
c.parts # => [#<Sprocket id: 1, name: "Fancy Sprocket">, #<Spring id: 1, name: "Fancy Spring">]
And you can get lists of the specific parts:
c.sprockets # => [#<Sprocket id: 1, name: "Fancy Sprocket">]
c.springs # => [#<Spring id: 1, name: "Fancy Spring">]
All of the above is implemented in this rails app.
The requirement is: Parts within the same container must have unique names.
Given the above models, how do you validate a Part such that it is scoped to its container? Rails' built-in uniqueness validator lets you set a :scope => [column_name], but that won't work in this case because there is no container_id column on our Part model.
My first thought was to have a custom validator on the Part abstract class that fails if self.container.parts.any?{|p| p.name == self.name } or something similar. However, at the time the validation is run, the container association doesn't yet exist, so we can't travel back that way.
The test/unit/part_assignment_test.rb file has several simple tests that demonstrate the desired behavior. One test is failing since I don't know how to implement the needed validation.
Run rake test to run the tests.
Any ideas?
- ryanb suggests moving the validation into PartAssignment instead of Part. See this branch.