belongs_to does not set foreign key depending object save order

[artworx]

Reproduced using the specs in spec/model/model_spec.cr

1, if User is saved after belogs_to is assigned it triggers an exception

temporary do
  reinit

  u = User.new({first_name: "John"})
  post = Post.new({user: u, title: "some post" })

  u.save!
  post.save! # Exception 

  post.user_id.should eq(u.id)
end

This fails with null value in column "user_id" violates not-null constraint, the SQL is

INSERT INTO "model_users" ("first_name", "updated_at", "created_at") VALUES ('John', '2020-03-20 11:27:14.096 +00:00', '2020-03-20 11:27:14.096 +00:00') RETURNING *
INSERT INTO "model_posts" ("title") VALUES ('some post') RETURNING *

2, it works when only Post is saved

temporary do
  reinit
  u = User.new({first_name: "John"})
  post = Post.new({user: u, title: "some post" })

  post.save!
  post.user_id.should eq(u.id) # true
end

INSERT INTO "model_users" ("first_name", "updated_at", "created_at") VALUES ('John', '2020-03-20 11:27:13.991 +00:00', '2020-03-20 11:27:13.991 +00:00') RETURNING *
INSERT INTO "model_posts" ("title", "user_id") VALUES ('some post', 1) RETURNING *

3, it works when User is saved before getting assigned to Post

temporary do
  reinit
  u = User.new({first_name: "John"})
  u.save!

  post = Post.new({user: u, title: "some post" })
  post.save!

  post.user_id.should eq(u.id)
end

INSERT INTO "model_users" ("first_name", "updated_at", "created_at") VALUES ('John', '2020-03-20 11:27:13.991 +00:00', '2020-03-20 11:27:13.991 +00:00') RETURNING *
INSERT INTO "model_posts" ("title", "user_id") VALUES ('some post', 1) RETURNING *