My client project uses a polymorphic relationship between several models in an effort to create a flexible system of associations.
However, I realized that this system was too flexible because it did not enforce the relationships as expected.
Our base
Here’s the domain we’ll be working with in this tutorial. The important thing to
note is that a product has_many :pictures and an employee has_one
:picture.
class Employee < ApplicationRecord
has_one :picture, as: :imageable
end
class Product < ApplicationRecord
has_many :pictures, as: :imageable
end
class Picture < ApplicationRecord
belongs_to :imageable, polymorphic: true
end
The problem
I’ve previously written about the limitations of a has_one relationship, and
this is no different. As you can see, it’s still possible to associate more than
one picture with an employee.
employee = Employee.last
employee.create_picture
Picture.create(imagable: employee)
Picture.where(imagable: employee).count
# => 2
A naïve solution
In the previous article we solved this by creating a unique index. Since
we’re working with a polymorphic relationship, we’ll need to make this index on
the imageable columns.
class AddContstraintToPictures < ActiveRecord::Migration[7.1]
def up
add_index :pictures, [:imageable_type, :imageable_id],
unique: true,
name: "by_employee"
end
def down
remove_index :pictures, name: "by_employee"
end
end
Then, we can compliment the unique index by adding a validates_uniqueness_of validation.
--- a/app/models/picture.rb
+++ b/app/models/picture.rb
@@ -1,3 +1,5 @@
class Picture < ApplicationRecord
belongs_to :imageable, polymorphic: true
+
+ validates_uniqueness_of :imageable_type, scope: :imageable_id
end
However, this approach is too heavy-handed. Although it prevents an employee
from having more than one picture, it also prevents a product from having
more than one picture.
product = Product.last
product.pictures.create!
picture = product.pictures.build
picture.valid?
=> false
picture.errors.messages
=> {:imageable_type=>["has already been taken"]}
An improved solution
What we need is a partial index. This allows us to conditionally enforce the
uniqueness constraint. In this case, we want to do this when the imageable_type
= "Employee".
--- a/db/migrate/20231123105601_add_contstraint_to_pictures.rb
+++ b/db/migrate/20231123105601_add_contstraint_to_pictures.rb
@@ -2,7 +2,8 @@ class AddContstraintToPictures < ActiveRecord::Migration[7.1]
def up
add_index :pictures, [:imageable_type, :imageable_id],
unique: true,
- name: "by_employee"
+ name: "by_employee",
+ where: "imageable_type = 'Employee'"
end
We can also add this conditional to the uniqueness validation by using the
conditional option.
--- a/app/models/picture.rb
+++ b/app/models/picture.rb
@@ -1,5 +1,6 @@
class Picture < ApplicationRecord
belongs_to :imageable, polymorphic: true
- validates_uniqueness_of :imageable_type, scope: :imageable_id
+ validates_uniqueness_of :imageable_type, scope: :imageable_id,
+ conditions: -> { where(imageable_type: "Employee") }
end
Wrapping up
Although this solution enforces our conditional uniqueness constraint in both
the database and application, it’s not necessarily the most flexible solution.
If you introduce a new model with has_one :picture, as: :imageable, you’ll
need to modify the database index.
Instead, you might want to consider just leveraging the validation at the application level, knowing that it’s possible duplicate records could still be added.
