Rails uniqueness constraint on two column values in different models

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.