In order to write a unit test case for model in rails 4, Let's suppose that we have a model city.rb which has some validations and associations.

#Rails 4.1.14.2 & ruby-2.1.7

Model path: my_app/app/models/city.rb, like...

class City < ActiveRecord::Base
  has_many :users
  belongs_to :country
  validates_presence_of :name, :country_id, :code
 
  # with other statement
end

In Unit-Test case for a model we create its factory, in factories folder, named cities.rb as table name in db. This file having model attribute and with dummy values 

Path: my_app/test/factories/cities.rb

FactoryGirl.define do
 factory :city do |f|
   country_id  123
   name        'myCity'
   code        'mycode'
   status        true              
  end
end

In city.rb model, there are two model associations i.e. city belongs_to country and city has_many users. So we need to create factories files for country ans users.

Path: my_app/test/factories/countries.rb

FactoryGirl.define do
 factory :country do |f|
    name           "India"
    code            'IN'
 end
end

Path: my_app/test/factories/users.rb

FactoryGirl.define do
 factory :user do |f|
  first_name          'name'
  last_name           'last'
  email               'fake@user.com'
  password            'password'
  gender              'Male'
  country_id          1
  city_id             12
 end
end

Now create a file in units folder named city_test.rb for testing its association ans validations

Path: my_app/test/unit/city_test.rb

require 'test_helper.rb'

class CityTest < ActiveSupport::TestCase

  test "validations of model" do
    city_obj = build(:city,name: "")
    assert_equal city_obj.save,false, "Validate presence of name"

    city_obj = build(:city,country_id: "")
    assert_equal city_obj.save,false, "Validate presence of country_id"
    
    city_obj = build(:city,code: "")
    assert_equal city_obj.save,false, "Validate presence of code"
  end

  def unit_test_for_has_many_user_association
    city = create(:city)
    u1 = create(:user,city_id: city.id,email: 'e1@unit.com')       
    u2 = create(:user,city_id: city.id,email: 'e2@test.com')
    
    assert_equal city.users, [u1,u2]
  end

Run your Test Case using command ...
ruby -I "lib:test" test/units/city_test.rb

Output:

Started
.

Finished in 0.589748669 seconds.

2 tests, 4 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
0% passed