Rails ActiveModel designing belongs_to and has_many with single class

Question

I need some help modeling my models and controller. Here is what I want to achieve:

I want to have a devise user named User (as usual) and a second model named Project. A Project should belong to a single User and at the same time should have many participants. The participants in a project should also be users (with devise registration/login) but the user, that created the project should not be able to participate. So far, so good. Here comes the tricky part: In my controller I want to be able to write:

def participate
  p = Project.find(id: params[:id])
  p.participants << current_user unless p.participants.includes?(current_user) && !p.user_id.equal(current_user.id)
  if p.save
    redirect_back
  else
    render :project
  end
end

This doesn't work because p.participants is not an array and the query (I tried it in rails console) does not check my n:m table. Here is my current model setup:

class Project < ApplicationRecord
  before_validation :set_uuid, on: :create
  validates :id, presence: true

  belongs_to :user
  has_and_belongs_to_many :participants, class_name: "User"
end

class User < ApplicationRecord
  before_validation :set_uuid, on: :create
  validates :id, presence: true

  devise :database_authenticatable, :registerable,
         :recoverable, :rememberable, :validatable

  has_and_belongs_to_many :projects
end

Finally my migrations:

class CreateProjects < ActiveRecord::Migration[6.0]
  def change
    create_table :projects, id: false do |t|
      t.string :id, limit: 36, primary_key: true
      t.string :title
      t.belongs_to :user, index: true, foreign_key: true, type: :uuid
      t.datetime :published_at

      t.timestamps
    end
  end
end

class CreateJoinTableProjectsUsers < ActiveRecord::Migration[6.0]
  def change
    create_join_table :users, :projects do |t|
      t.index :project_id
      t.index :user_id
    end
  end
end

Answer 1

It is better to use has_many: through instead of has_and_belongs_to_many. This allows you to write cleaner code for validation.

1. Remove has_and_belongs_to_many from User and Project models

2. Add has_many :through to User and Project models

   rails g model UserProject user:references project:references
   rails db:migrate
   
   class User < ApplicationRecord
     ..
     has_many :user_projects
     has_many :projects, through: :user_projects
     ..
   end
   
   class Project < ApplicationRecord
     ..
     has_many :user_projects
     has_many :participants, through: :user_projects, source: 'user'
     ..
   end
   
   class UserProject < ApplicationRecord
     belongs_to :user
     belongs_to :project
   end
   

3. Add validation to UserProject model

   class UserProject < ApplicationRecord
     belongs_to :user
     belongs_to :project
   
     validate :check_participant
   
     private
     def check_participant
       return if project.participants.pluck(:id).exclude?(user.id) && project.user != user
       errors.add(:base, 'You cannot be participant')
     end
   end
   

4. Update participate method

   def participate
     p = Project.find(id: params[:id])
     begin
       p.participants << current_user
       redirect_back
     rescue ActiveRecord::RecordInvalid => invalid
       puts invalid.record.errors
       render :project
     end
   end