Validates uniqueness of has_many relationship in Ruby on Rails

Validates uniqueness of `has_many` relationship in Ruby on Rails.

Here is the mockup of users & subjects, the relationship is users can create more than one subjects.

But we need not able to create a duplicate record of every individual user as like if a user John Doe creates as a subject Ruby once for this user not fair to create Ruby again, but Ruby as a subject can create another user but only once not a duplicate.

See the below mockup

If you concentrate this image you can’t find the subject name use a duplicate for a user.

Now we can see our models like user.rb & subject.rb

#=> user.rb
class User < ApplicationRecord
    has_many :subjects
end

#=> subject.rb
class Subject < ApplicationRecord
    belongs_to :user
end

Now if you create subjects it will take duplicates of a subject, so we add validates_uniqueness_of :name then it will take only once & not able to create multiple users subject as Ruby.

Now we can change our subject model slightly, see the below:

#=> subject.rb
class Subject < ApplicationRecord
    belongs_to :user
    validates_uniqueness_of :name, scope: :user_id
end

we added here scope: :user_id & that’s why it’s can take name unique based on user, a user can’t create a duplicates of name.