Rails uniqueness constraint on two column values in different models - ruby-on-rails

Question

I want to know the best way to have an uniqueness constraint enforced on two related model attributes in rails that are both no primary keys
class Parent > ApplicationRecord
has_many :children
:name
end
class Child > ApplicationRecord
:name
end
I want to enforce that (parent.name, child.name) is unique for every parent. e.g.
(parent1, child1) and (parent2, child1) is allowed
(parent1, child1) and (parent1, child1) is a violation
Ideally, I would enforce this in Postgres, however I have only seen the option to add uniqueness constraints on multiple columns of the same table.
Alternatively, I have written a custom validator for rails that does what I want, but this is cumbersome. There needs to be a better solution...
For completeness, here is the constraints validator which requires one to add a children function to a model returning the list of children.
class NamePairValidator < ActiveModel::Validator
def validate(record)
record.children.values.each do |model_children|
names = model_children.to_a.collect {|model| model.name}
if (names.select{|name| names.count(name) > 1 }.size > 0)
record.errors[:name] << 'Path leading to this resource has no unique name'
end
end
end
end
(in Parent.rb)
def children
{children: :children}
end
Migrations:
class CreateDomains < ActiveRecord::Migration[5.0]
def change
create_table :domains do |t|
t.string :name
t.string :domain_type
t.timestamps
end
end
end
class CreateSubjects < ActiveRecord::Migration[5.0]
def change
create_table :subjects do |t|
t.string :name
t.string :subject_type
t.timestamps
end
end
end
class CreateJoinTableDomainSubject < ActiveRecord::Migration[5.0]
def change
create_join_table :domains, :subjects do |t|
t.index [:domain_id, :subject_id]
t.index [:subject_id, :domain_id]
end
end
end

Answer 1

I just used similar one in my code
validates :child_name, uniqueness: { scope: :parent_id }
More..
(i) https://apidock.com/rails/ActiveRecord/Validations/ClassMethods/validates_uniqueness_of
(ii) Validate uniqueness of multiple columns

Answer 2

Insipered by the-has-many-through-association of the official doc of ruby on rails:
class CreateAppointments < ActiveRecord::Migration[5.0]
def change
create_table :domains do |t|
t.string :name, null: false
t.string :domain_type
t.timestamps
end
create_table :subjects do |t|
t.string :name, null: false
t.string :subject_type
t.timestamps
end
create_table :fields do |t|
t.belongs_to :domain, index: true
t.belongs_to :subject, index: true
t.timestamps
end
end
end
Note
I took the initative to rename your model JoinTableDomainSubject by Field to be more readable.
I also force name field not be nil to check uniqueness.
(adding null: false in migrations files and validates :name, presence: true in both models)
Now the dedicated classes:
class Subject < ApplicationRecord
has_many :fields
has_many :domains, through: :fields
validates :name, presence: true
end
class Domain < ApplicationRecord
has_many :fields
has_many :subjects, through: :fields
validates :name, presence: true
end
class Field < ApplicationRecord
belongs_to :domain
belongs_to :subject
validate :domain_and_subject_names_uniqueness
private
def domain_and_subject_names_uniqueness
if class.includes(:domain, subject)
.where(domain: { name: domain.name }, subject: { name: subject.name })
.exists?
errors.add :field, 'duplicity on names'
end
end
end
Since the models are associated, I can use Field.first.domain to access Domain model of a given Field and vice versa.