Does Rails has a way to validate if the foreign key value exists in an optional relationship? - ruby-on-rails-5.1

Question

I am using Rails version 5.1.3.
Does Rails have a way to validate if the foreign key value exists in an optional relationship?
I have the next model:
class Post < ApplicationRecord
belongs_to :category, optional: true
validates :category, presence: true, allow_nil: true
end
This is the migration:
class CreatePosts < ActiveRecord::Migration[5.1]
def change
create_table :posts do |t|
t.string :name, limit: 100
t.references :category, foreign_key: true, null: true
t.timestamps
end
end
end
These are the cases:
# Case 1
p1 = Post.new({})
p1.save #Working as I expected ... record Inserted
# Case 2
p2 = Post.new({category_id: 3}) # A category with id 3 exists
p2.save #Working as I expected ... record Inserted
# Case 3
p3 = Post.new({category_id: 30}) # A category with id 30 does not exists.
p3.save # Not working as I expected
In the case 3 I was expecting an Active Record validation error something like This Category does not exists but instead of I get
an SQL message:
INSERT INTO "posts" ("category_id", "created_at", "updated_at") VALUES (?, ?, ?) [["category_id", 10], ["created_at", "2017-09-26 00:59:47.645185"], ["updated_at", "2017-09-26 00:59:47.645185"]]
ActiveRecord::InvalidForeignKey: SQLite3::ConstraintException: FOREIGN KEY constraint failed: INSERT INTO "posts" ("category_id", "created_at", "updated_at") VALUES (?, ?, ?)

Answer 1

This could help:
validates :category,
presence: true,
if: -> { category_id.present? }

Answer 2

The database is enforcing foreign key integrity, which just means that it will not allow you to associate things to another thing that does not exist. This is expected behavior.
If you want to do something if the association doesn't exist, you should do a find first before attempting to create the association.
The optional validation you set up for the foreign key relationship just means that a relationship is not required. In other words, you can create posts that are not associated with categories. However, that does not mean the database will be happy if you attempt to form an association with categories that do not exist. Rails is cool whether or not you provide an association or not, but the moment it attempts to do something that violates rules set by the database, then the database has every right to complain!