Error in the association of two models. (# must exists) - ruby-on-rails

Question

I want to be able to choose an author when creating a post. I have a Post and Author model. In the author's model, I prescribed has_many :posts, and in the Post model belongs_to :author. In the form view, created a list of authors <%= form.collection_select(:author_id, Author.all, :id, :last_name, class:'form-control' ) %>. In the post controller I prescribed:
def post_params
params.require(:post).permit(:name, :title, :content, :picture, :author_id)
end
schema.rb:
create_table "posts", force: :cascade do |t|
t.string "name"
t.string "title"
t.text "content"
t.datetime "created_at", precision: 6, null: false
t.datetime "updated_at", precision: 6, null: false
t.integer "author_id"
t.index ["author_id"], name: "index_posts_on_author_id"
end
But when creating a post, I get an error:
Author must exist
logs:
Started POST "/posts" for ::1 at 2019-11-18 00:51:40 +0200
Processing by PostsController#create as HTML
Parameters: {"authenticity_token"=>"68iOabMzXQPuvC83f5Oe3kQEl5rFeQLQNCuMjvtRZgfdArdvMs79gKs8MeTNzBcsN0dRjTdRVCFkIrvZqixY1g==", "post"=>{"author_id"=>"2", "name"=>"SomeName", "title"=>"SomeTitle", "content"=>"SomeContent"}, "commit"=>"Submit"}
Rendering posts/new.html.erb within layouts/application
[1m[36mAuthor Load (0.2ms)[0m [1m[34mSELECT "authors".* FROM "authors"[0m
↳ app/views/posts/_form.html.erb:16
Rendered posts/_form.html.erb (Duration: 8.2ms | Allocations: 2071)
Rendered posts/new.html.erb within layouts/application (Duration: 9.1ms | Allocations: 2157)
Completed 200 OK in 34ms (Views: 30.4ms | ActiveRecord: 0.2ms | Allocations: 13745)```

Answer 1

The Author must exist error indicates that your new post is failing a validation: the presence of a value for the author_id attribute which links a post to an author. This validation comes bundled by default with the belongs_to association helper method (and can be disabled as per Simon Franzen's comment above: belongs_to: model, optional: true.
Since you're permitting the author_id key in post_params, the weak link in the chain is either your post#create controller action or your _form.html.erb view. A couple of suggestions for post#create, and _form.html.erb, respectively:
def create
post = Post.create(post_params)
redirect_to post
end
<%= form_with model: #post do |form| %>
...
<%= form.label :author %>
<%= form.collection_select :author_id, Author.all, :id, :name %>
...
<%= form.submit "Create Post" %>
<% end %>
We need more of the code for your post controller actions and form view to give a more precise answer.