How to write validation for checking ID?

Question

Ruby on Rails

There are two models: User and Post.
The posts table has a user_id field. How to check it when adding a post, they say, is there a record with such an ID in the users table or is it absent?
Yes validates_associated, but this helper seems to perform a slightly different task.
Is this really the only true option?

Answer 1

@user = User.find(params[:user_id])
@post = @user.posts.build(post_params)
@post.save