jBoxer

I change the directions of small pieces of metal for a living.

Using acts_as_list in a polymorphic scope

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If you’re using acts_as_list in a polymorphic scope, you need to define the scope a little bit differently to make everything work right.

Here’s the non-polymorphic example from acts_as_list’s README:

acts_as_list in a normal (non-polymorphic) scope
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class TodoItem < ActiveRecord::Base
  belongs_to :todo_list
  acts_as_list scope: :todo_list # Note the single-item scope
end

We pass :todo_list as the scope (which acts_as_list converts to the todo_list_id column).

Now, let’s say we’re making a polymorphic Picture model (like in the Polymorphic Associations section of the Ruby on Rails Guides), and we want pictures to be sortable and reorderable. Here’s the right way to do it:

acts_as_list in a polymorphic scope (the RIGHT way)
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class Picture < ActiveRecord::Base
  belongs_to :imageable, polymorphic: true
  acts_as_list scope: [:imageable_id, :imageable_type]
end

Note how, in the Picture model, the scope we pass to acts_as_list is an array of the two columns that make up the polymorphic association. If we were to follow our template from TodoItem and do it this way:

acts_as_list in a polymorphic scope (the WRONG way)
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class Picture < ActiveRecord::Base
  belongs_to :imageable, polymorphic: true
  acts_as_list scope: :imageable
end

Everything would run correctly, but imageables with the same ID and different types would have some problems. Specifically, determining which pictures are first and last:

Bugs caused by using acts_as_list in a polymorphic scope the WRONG way
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class Picture < ActiveRecord::Base
  belongs_to :imageable, polymorphic: true
  acts_as_list scope: :imageable
end

class Employee < ActiveRecord::Base
  has_many :pictures, as: :imageable
end

class Product < ActiveRecord::Base
  has_many :pictures, as: :imageable
end

employee1 = Employee.create      # employee1 is an Employee with id=1
employee2 = Employee.create      # employee2 is an Employee with id=2
product1 = Product.create        # product1 is a Product with id=1

employee1.pictures.create        # Create a new picture on employee 1
employee2.pictures.create        # Create a new picture on employee 2
product1.pictures.create         # Create a new picture on product 1

puts employee1.pictures[0].last? # false, unexpected
puts employee2.pictures[0].last? # true
puts product1.pictures[0].last?  # true

Since each imageable has only one Picture, last? (an acts_as_list method that checks if the item is the last one in scope) should return true for all three. But, the first line says false. Since we’re only telling acts_as_list to scope on :imageable (which it converts to imageable_id), it has no way of telling between the Employee with an id of 1 and the Product with an id of 1. So, it thinks that Employee 1’s picture belongs to the same record as Product 1’s picture, and thus, thinks that only Product 1’s picture is last?.

If you tell acts_as_list the full story, it won’t get confused:

Correct results from using acts_as_list in a polymorphic scope the RIGHT way
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class Picture < ActiveRecord::Base
  belongs_to :imageable, polymorphic: true
  acts_as_list scope: [:imageable_id, :imageable_type]
end

class Employee < ActiveRecord::Base
  has_many :pictures, as: :imageable
end

class Product < ActiveRecord::Base
  has_many :pictures, as: :imageable
end

employee1 = Employee.create      # employee1 is an Employee with id=1
employee2 = Employee.create      # employee2 is an Employee with id=2
product1 = Product.create        # product1 is a Product with id=1

employee1.pictures.create        # Create a new picture on employee 1
employee2.pictures.create        # Create a new picture on employee 2
product1.pictures.create         # Create a new picture on product 1

puts employee1.pictures[0].last? # true
puts employee2.pictures[0].last? # true
puts product1.pictures[0].last?  # true

Now that acts_as_list knows to look at both the imageable_id and the imageable_type, it can tell the difference between Employee 1 and Product 1, and returns true for all three last?s, as expected.

Posted by Jake Boxer

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