Searchkick, typeahead.js and Elasticsearch issues - ruby-on-rails

Question

I am trying to implement an autocomplete search bar that searches posts in my Rails 4.2 app. I have elasticsearch installed, searchkick gem, and typeahead.js for autocomplete on the frontend.
To that end, I followed a tutorial (https://shellycloud.com/blog/2013/10/adding-search-and-autocomplete-to-a-rails-app-with-elasticsearch) and have the following things set up:
In my routes.rb I have set up the collection route like so:
resources :posts do
collection do
get :autocomplete
end
resources :attachments
end
In my Post.rb model:
searchkick autocomplete: ['title', 'excerpt']
In my posts_controller.rb
def index
if params[:query].present?
#posts = Post.search(params[:query])
end
end
def autocomplete
render json: Post.search(params[:query], autocomplete: true, limit: 10).map do |post|
{title:post.title, excerpt: post.excerpt}
end
end
In my posts.js
$(document).ready(function() {
var posts = new Bloodhound({
datumTokenizer: Bloodhound.tokenizers.obj.whitespace('title'),
queryTokenizer: Bloodhound.tokenizers.whitespace,
prefetch: "/posts/autocomplete?query=%QUERY",
remote: {
url: "/posts/autocomplete?query=%QUERY"
}
});
$('#query').typeahead({
name: 'posts',
display: 'title',
source: posts
});
})
Finally, in my view index.html.haml I have:
=form_tag(posts_path, method: :get) do
=text_field_tag(:query, params[:query], autocomplete: 'off', class: 'typeahead')
%input{:type=>"submit", :value=> t('search')}
When I type something in the search box, I get this:
GET XHR http://localhost/posts/autocomplete [HTTP/1.1 400 Bad Request 177ms]
I can't tell what is going on. For what its worth, I have tried hitting http://localhost/posts/autocomplete.json?query=something and it works in that it will return the matching result. I have also tried replacing autocomplete with autocomplete.json in the JS I posted above.
Still, I get the same error in the console (with autocomplete.json in place of autocomplete)
Can you help?