Store value from another table form in rails - ruby-on-rails

Question

I have a users table and games table. Games table has user_id. The help I want is how can I change/enter the value of city of birth from the game's form which is a field in the user table. I am using the try() method to display the value of city of birth from the user table in the game's form.
user.rb
has_many :games, dependent: :destroy
game.rb
belongs_to :user
_form.html.erb(game)
<div class="custom-hidden field">
<%= form.label :user_id %>
<%= form.number_field :user_id %>
</div>
<div class="field">
<%= form.label :city_of_birth, "User City of Birth" %>
<%= form.text_field :city_of_birth, :value => #user.try(:city_of_birth) %>
</div>
<div class="field">
<%= form.label :game_name %>
<%= form.text_field :game_name %>
</div>

Answer 1

I'm going to answer this question assuming that this form is meant to create a Game, which belongs_to :user, since there is a user_id in the form, and that user.city_of_birth is a string.
The traditional way to do this would be to use theaccepts_nested_attributes_for feature of Rails.
https://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html
However, I would strongly suggest that you consider writing a form object to handle this, so that the responsibility for validating this mixed-model form is held cleanly in one place. I suggest using a gem like Reform to make this process easier.
# app/forms/games/new_game_form.rb
class Games::NewGameForm < Reform::Form
property :user_id, validates: { presence: true }
property :city_of_birth, virtual: true, validates: { presence: true }
property :game_name, validates: { presence: true }
def save
Game.transaction do
user.update!(city_of_birth: city_of_birth)
Game.create(user: user, game_name: game_name)
end
def user
#user ||= User.find(user_id)
end
end
This form object can then be used in place of the Game instance.
# GamesController
def new
#form = Games::NewGameForm.new(Game.new)
end
<!-- new.html.erb -->
<%= form_with #form, url: games_path, method: :post do |f| %>
<!-- your form fields -->
<% end %>
Please note that it appears very odd that you're accepting a user_id in the form, but also reading the city_of_birth from #user. If the #user for whom the game is being created is already known (perhaps the signed in user), then it's useless (and a security risk) to accept the ID in the form - you can simply use the same method that was used to set #user.