Should I write these Test Cases?

Here's the schema of the model I want to test:

create_table "retreats", force: :cascade do |t|
  t.string   "title"
  t.string   "tagline"
  t.string   "type_of"
  t.datetime "created_at",                 null: false
  t.datetime "updated_at",                 null: false
  t.string   "description"
  t.string   "schedule"
  t.boolean  "available",   default: true
end

Here's the Retreat model:

class Retreat < ApplicationRecord
  TYPES_OF_RETREATS = ['individual', 'group']
  validates :title, presence: true
  validates :type_of, presence: true, inclusion: {in: TYPES_OF_RETREATS,
    message: "%{value} is not a valid type."}

  has_many :testimonials, dependent: :destroy
  has_many :images, dependent: :destroy
  has_and_belongs_to_many :dates, class_name: "RetreatDate", foreign_key: 
   'retreat_id', association_foreign_key: 'retreat_date_id'
end

These are the test cases I've written:

  test "retreat should not save without a title" do
    retreat = retreats(:no_title)
    assert_not retreat.save, "Saved a retreat without a title"
  end

  test "retreat should not save without a type" do
    retreat = retreats(:no_type)
    assert_not retreat.save, "Saved a retreat without a type"
  end

  test "retreat can have a tagline, description, schedule and available" do
    retreat = retreats(:all_attributes)
    assert retreat.save, "Retreat failed to save"
  end

  test "retreat type should be from the provided list" do
    retreat = retreats(:invalid_type)
    assert_not retreat.save, "Some other retreat got saved. It shouldn't 
     have gotten saved."
  end

  test "retreat can have many testimonials" do
    retreat = retreats(:one)
    retreat.testimonials << Testimonial.new(statement: 'this is my 
      testimonial', participant_name: 'abc')
    assert retreat.save, "Retreat did not save with the testimonials."
  end

  test "retreat can have many dates" do
    retreat = retreats(:one)
    retreat.dates.create({date: '02-08-2012'})
    retreat.dates.create({date: '02-08-2013'})
    assert retreat.save, "Retreat with multiple dates is not saving"
    assert_equal(2, retreat.dates.length, "Retreat isn't saving multiple 
       dates.")
  end

I am looking for advice on what kind of test cases should I write tests for. I feel that some of my test cases are unnecessary. Like test cases for validation make sense but testing if I can add multiple testimonials makes me uncomfortable.

What are the best practises for writing unit tests? And how can I write better unit tests?

Thanks :)