rsvp button (users can only rsvp once) - ruby-on-rails

Question

So currently users can rsvp multiple times for the same event. That is obviously problematic for my website so I want it that a single user can only rsvp once for a specific post/event. After they have made their Rsvp I want the rsvp button to disappear. Heres is how my code is looking.
show.html.erb
id="notice"><%= notice %></p>
<p>
<strong>Date:</strong>
<%= #post.date %>
</p>
<p>
<strong>Name:</strong>
<%= #post.name %>
</p>
<p>
<strong>User_id:</strong>
<%= #post.user_id %>
</p>
<p><strong>Address:</strong> <%= #post.address %></p>
<p>
<strong>Description:</strong>
<%= #post.description %>
</p>
<p>
<strong>registered:</strong>
<%=#post.users.length%>
</p>
<% if current_user == #post.user %>
<%= link_to 'Edit', edit_post_path(#post) %> |
<%end%>
<%= link_to 'Back', posts_path %>
<div class="rsvp"><%= button_to "Rsvp now", rsvps_post_path(#post), class: "btn btn-primary" %></div>
<div class="map"><%= image_tag "http://maps.googleapis.com/maps/api/staticmap?center=#{#post.latitude},#{#post.longitude}&markers=#{#post.latitude},#{#post.longitude}&zoom=12&size=450x400&sensor=false&key=AIzaSyCKzKMEhSNgwSXf7WV71pHWgzdpMkPn8W4",
class: 'img-fluid img-rounded', alt: "#{#post} on the map"%>
</div>
post.rb
class Post < ApplicationRecord
belongs_to :user
geocoded_by :address
after_validation :geocode, if: ->(obj){ obj.address.present? and obj.address_changed? }
reverse_geocoded_by :latitude, :longitude
after_validation :reverse_geocode
has_many :rsvps
has_many :users, through: :rsvps
validates :name, presence: true
user.rb
class User < ApplicationRecord
# Include default devise modules. Others available are:
# :confirmable, :lockable, :timeoutable and :omniauthable
devise :database_authenticatable, :registerable,
:recoverable, :rememberable, :trackable, :validatable
has_many :posts
has_many :rsvps
has_many :posts, through: :rsvps
validates :email, presence: true
rsvp.rb
class Rsvp < ApplicationRecord
belongs_to :user
belongs_to :post
end
#rsvp migration
class Rsvp < ApplicationRecord
belongs_to :user
belongs_to :post
end
So I've looked around in Stackoverflow and googled for a bit but I'm at a loss. I would really appreciate an answer that would solve this issue. I just want the rsvp button to show users who havent rsvped for the specific post/event.

Answer 1

The most important thing to me is a database constraint. You want to instruct the database to throw an error in those cases.
In a migration
add_index :rsvps, [:user_id, :post_id], unique: true
And a validation that reflect that, just to make rails aware of this
validates :user_id, uniqueness: { scope: :post_id }
Now we're sure that no more than one pair some_user_id, some_post_id is present in the database.
Now, let's instruct the view to not display the button in those cases
<% unless Rsvp.exists?(post: #post, user: #post.user) %>
<div class="rsvp">
<%= button_to "Rsvp now", rsvps_post_path(#post), class: "btn btn-primary" %>
</div>
<% end %>
I'd move that exists query in the action and use just a boolean here, this is just demonstrative.