test_task

after.rb

#
# refactored
#
class User < ApplicationRecord
  has_many :user_interests
  has_many :interests, through: :user_interests

  has_many :user_skills
  has_many :skills, through: :user_skills
end

class Interest < ApplicationRecord
  has_many :user_interests
  has_many :users, through: :user_interests
end

class Skill < ApplicationRecord
  has_many :user_skills
  has_many :users, through: :user_skills
end

# many-to-many models
class UserInterest < ApplicationRecord
  belongs_to :users
  belongs_to :interests
end

class UserSkill < ApplicationRecord
  belongs_to :users
  belongs_to :skills
end
#

class Users::Create < ActiveInteraction::Base
  hash :params do
    array :interests
    array :skills
    string :name, :patronymic, :email, :nationality, :country, :gender
    integer :age
  end

  validate :gender_valid?
  validate :age_valid?
  validate :user_exists?

  def execute
    user_full_name = [ params["surname"], params["name"], params["patronymic"] ].compact.join(" ")
    user_params = params.merge(user_full_name: user_full_name)
                        .except(:interests, :skills)

    ActiveRecord::Base.transaction do
      user = User.create!(user_params)

      Interest.where(name: params["interests"]).each do |interest|
        user.interests.create!(name: interest)
      end

      params["skills"].split(",").uniq.each do |skill|
        user.skills << Skill.find_or_create_by!(name: skill)
      end
    end
  end

  private

  def age_valid?
    errors.add(:age, "You're too old or too young to use register here!") if params["age"] <= 0 || params["age"] > 90
  end

  def gender_valid?
    if params["gender"] != "male" && params["gender"] != "female"
      errors.add(:gender, "Unknown gender!")
    end
  end

  def user_exists?
    errors.add(:gender, "User is exists!") if User.exists?(name: params["name"])
  end
end

before.rb

#
# THIS IS A BEFORE REFACTORING VARIANT!!!!!!
#
class User < ApplicationRecord
  has_many :interests
  has_many :skills, class_name: 'Skil'
end

class Interest < ApplicationRecord
  has_many :users
end

class Skil < ApplicationRecord
  has_many :users
end

class Users::Create < ActiveInteraction::Base
  hash :params

  def execute
    #don't do anything if params is empty
    return unless params['name']
    return unless params['patronymic']
    return unless params['email']
    return unless params['age']
    return unless params['nationality']
    return unless params['country']
    return unless params['gender']
    ##########
    return if User.where(email: params['email'])
    return if params['age'] <= 0 || params['age'] > 90
    return if params['gender'] != 'male' or params['gender'] != female

    user_full_name = "#{params['surname']} #{params['name']} #{params['patronymic']}"
    user_params = params.except(:interests)
    user = User.create(user_params.merge(user_full_name))

    Intereset.where(name: params['interests']).each do |interest|
      user.interests = user.interest + interest
      user.save!
    end

    user_skills = []
    params['skills'].split(',').each do |skil|
      skil = Skil.find(name: skil)
      user_skills =  user_skills + [skil]
    end
    user.skills = user_skills
    user.save
  end
end