group.rb

class Group < ActiveRecord::Base
  has_many :memberships
  has_many :users, :through => :memberships
  belongs_to :owner, class_name: 'User'
  validates :owner, presence: true
  validates :name, presence: true, length: { minimum: 6, maximum: 255 }
  validates :description, length: { maximum: 255 }, allow_blank: true

end

group_test.rb

require 'test_helper'

class GroupTest < ActiveSupport::TestCase

  def setup
    @group = Group.new(name: 'test group', description: 'test group description', owner: users(:michael))
    @group.users << users(:michael)
    @user = users(:michael)
  end

  test "should be valid" do
    assert @group.valid?
  end

  test "should have an owner" do
    @group.owner = nil
    assert_not @group.valid?
  end

  test "owner should be a member" do
    assert @group.users.include?(@group.owner)
  end

  test "name should be present" do
    @group.name = "    "
    assert_not @group.valid?
  end

  test "name should not be too short" do
    @group.name = 'a' * 5
    assert_not @group.valid?
  end

  test "name should not be too long" do
    @group.name = 'a' * 256
    assert_not @group.valid?
  end

  test "description can be empty" do
    @group.description = ""
    assert @group.valid?
  end

  test "description should not be too long" do
    @group.description = 'a' * 256
    assert_not @group.valid?
  end

  test "group has an owner" do
    assert @group.owner
  end

  test "owner has a group" do
    # byebug
    assert @user.groups.any?
  end

end

membership.rb

class Membership < ActiveRecord::Base
  belongs_to :user
  belongs_to :group
  validates :user, presence: true
  validates :group, presence: true
end

minitest results from guard

12:29:55 - INFO - Running: test/models/group_test.rb
Run options: --seed 30869

# Running:

...F......

Finished in 0.705084s, 14.1827 runs/s, 14.1827 assertions/s.

  1) Failure:
GroupTest#test_owner_has_a_group [/Users/matthew/Work/Shadowland7/shadowland/test/models/group_test.rb:55]:
Failed assertion, no message given.

10 runs, 10 assertions, 1 failures, 0 errors, 0 skips