add user_id to new record

In your Controller's #new and #create methods, scope the post to be owned by the current_user (or whatever other scheme you may be using for your logged-in-user management).

def new
  @post = current_user.posts.build
end

def create
  @post = current_user.posts.create(post_params)
  ...
end

Now your relationship will take care of getting the correct user in there automatically, whether it's in-memory or in persistence.

NB: If this doesn't appear to work, then you haven't built your relationships correctly, and you need to fix that before proceeding.

Walter

Ok thanks And what about passing some params to build?

@post = current_user.posts.buil(artist_id: @artist.id, tour_id: @tour.id)