class Event < ApplicationRecord
...
  belongs_to :organization
...
end

class Organization < ApplicationRecord
...
  has_many  :events, :dependent => :destroy
...
end

FactoryGirl.define do
  factory :organization do
    organization_name { Faker::Company.name }
  end
  factory :invalid_organization, class: Organization do
    organization_name ''
  end
end

FactoryGirl.define do
  factory :event do
    event_description { Faker::Lorem.sentence(3) }
    host_name { Faker::Internet.domain_name }
    organization = build(:organization)
    organization_id = organization.id
  end
end

require 'rails_helper'

RSpec.describe Event, type: :model do
  it "has a valid factory" do
    event = build(:event)
    expect(event).to be_valid
  end
  it { is_expected.to validate_presence_of(:event_description) }
  it { is_expected.to validate_presence_of(:host_name) }
  it { is_expected.to validate_presence_of(:organization_id) }
  it { is_expected.to belong_to(:organization) }
end

/Users/levi/ror/events-handler/spec/factories/events.rb:6:in `block (2 levels) in <top (required)>': undefined method `id' for #<FactoryGirl::Declaration::Static:0x007febc03db798> (NoMethodError)

FactoryGirl.define do
  factory :event do
    event_description { Faker::Lorem.sentence(3) }
    host_name { Faker::Internet.domain_name }
    organization { build(:organization) }
  end
end