% rails test test/controllers/posts_controller_test.rb   
Running 5 tests in a single process (parallelization threshold is 50)
Run options: --seed 46393

# Running:

...E

Error:
PostsControllerTest#test_should_destroy_post:
ActiveRecord::InvalidForeignKey: SQLite3::ConstraintException: FOREIGN KEY constraint failed
    app/controllers/posts_controller.rb:38:in `destroy'
    test/controllers/posts_controller_test.rb:33:in `block (2 levels) in <class:PostsControllerTest>'
    test/controllers/posts_controller_test.rb:32:in `block in <class:PostsControllerTest>'

# add all has_many :YOUR_MODEL, :dependent => :destroy

class Post < ApplicationRecord
  belongs_to :user
  belongs_to :category
  has_many :comments, :dependent => :destroy
end