PostsControllerTest Error with Simple Ruby on Rails Program - ruby-on-rails

Question

Full Code on GitHub, https://github.com/Marium36/blog
I am taking online Ruby on Rails course that guides us to create simple blog application.
Running this on Ubuntu 18.04
Following commands work
> rails new blog
> cd blog
> rails generate scaffold Post title:string body:text
> rails generate scaffold Comment post:references body:text
> rake db:migrate
> rake routes
> rails server
As soon as I run this (per the course)
> rake db:test:load
> rake test
I get error
Run options: --seed 62159
# Running:
.E
Error:
PostsControllerTest#test_should_destroy_post:
ActiveRecord::InvalidForeignKey: SQLite3::ConstraintException: FOREIGN KEY constraint failed
app/controllers/posts_controller.rb:57:in `destroy'
test/controllers/posts_controller_test.rb:43:in `block (2 levels) in <class:PostsControllerTest>'
test/controllers/posts_controller_test.rb:42:in `block in <class:PostsControllerTest>'
rails test test/controllers/posts_controller_test.rb:41
............
Finished in 7.151961s, 1.9575 runs/s, 2.2371 assertions/s.
14 runs, 16 assertions, 0 failures, 1 errors, 0 skips

Answer 1

The error you're experiencing (Foreign Key constraint failed) is telling you that you have a foreign key constraint in your database that is not met when some particular piece of code is run.
What is a foreign key?*
In relational databases—like MySQL, PostgreSQL, or SQLite—you typically have tables of data that relate to one another in some way. It's very common for rows in those tables to reference one another using an ID, which is referred to as the “key”. You might have, as you do here, a comments table with a post_id column that references the id column of the posts table. In this case, the post_id is referencing the foreign key posts.id.
What is a constraint?
In relational databases, constraints provide a way to ensure the integrity of your data. A constraint can do things like ensure a price column is never negative (i.e. verify based on the data itself), ensure one value is lower than another in the same row, or verify that the data matches another record in another table entirely.
Putting it together: what is a foreign key constraint?
A foreign key constraint, if you haven't already put it together, is just a special type of constraint that ensures that the “foreign record”—the row in the table you're referencing—actually exists. In this case, comments.post_id has a foreign key constraint on posts.id, which ensures that a row can't be added to the comments table with a post_id that doesn't exist in the posts.id column.
So why did you get this error?
This is your migration creating the comments table:
class CreateComments < ActiveRecord::Migration[6.0]
def change
create_table :comments do |t|
t.references :post, null: false, foreign_key: true
t.text :body
t.timestamps
end
end
end
Note the foreign_key: true part! That's telling Rails to add a foreign key constraint between comments and posts on the foreign key.
This is the test that is failing:
test "should destroy post" do
assert_difference('Post.count', -1) do
delete post_url(#post)
end
assert_redirected_to posts_url
end
It's deleting a Post record, which tells us that there is a row in the comments table with a post_id column referencing the Post we're trying to remove in this test, hence the failure.
What can I do to fix it?
First, you probably want to add an inverse relationship to your Post model, like this:
class Post < ApplicationRecord
has_many :comments
Then you'll want to tell Rails how to handle associated records when you're removing a Post. You can do that with the dependent option, which takes a handful of options. I'll use :delete_all here, which tells Rails to use a single SQL query to delete all “dependent” comments (that is, comments that reference this Post) without loading them first:
class Post < ApplicationRecord
has_many :comments, dependent: :delete_all
end
With that change your test should succeed because it will have removed the comments as well as the post in the same transaction, preventing the foreign key constraint from failing.