Rails collection_select vs f.collection_select

Question

I can't understand the difference between the two. Can someone please explain the difference when using form_for?

Say you have this: <%= form_for(@post) do |f| %>

Examples- When would you use this?

<%= collection_select(:post, :author_id, Author.all, :id, :name_with_initial, prompt: true) %>

vs. use this?

<%= f.collection_select(:post, :author_id, Author.all, :id, :name_with_initial, prompt: true) %>

Rails Api

Answer 1

When you use f.some_form_helper the helper will already know the name of the model you want to make the field name for. This way you can drop that :post argument. form_for(@post) gives you the f form builder object that knows what model the form is for.

With the regular collection_select (or any other helper with f.) you have to pass in, as the first argument, the name of the model the field is for.

Your example is a bit off because you passed in the same arguments to both. f.collection_select doesn't need the :post.

This is correct use of the non f. helper:

<%= collection_select(:post, :author_id, Author.all, :id, :name_with_initial, prompt: true) %>

This is a corrected way to use the f. helper:

<%= f.collection_select(:author_id, Author.all, :id, :name_with_initial, prompt: true) %>

the f object has a reference back to the model you passed in to form_for via f.object. This is how it knows to call collection_select(:post, ...) under the hood.