Runs just fine with "ruby test.rb" under ruby 2.5

test.rb

require 'factory_bot'

class Training
  attr_accessor :instructor
  def save!; end
end

class User
  attr_accessor :role
  def save!; end
end

class Activity
  attr_accessor :name
  def save!; end
end

class Role
  attr_accessor :activities
  def save!; end
end

FactoryBot.define do
  factory :training do
    transient do
      assigned_instructor { create(:user) }
    end

    instructor  { assigned_instructor }
  end

  factory :user do
    role  { create(:role, activities: [create(:activity, name: 'activity')]) }
  end

  factory :activity do
  end

  factory :role do
  end
end

training = FactoryBot.create :training
p training.inspect