Work with two tables in one controller

I have tried in so many ways and can't get it to work. I did something
yesterday that made the app have an redirect loop (too many redirects)
and it took me over an hour just to fix it. So I'm just going to start
all over again.

My initial question had to do with other models but it's basically the
same. I need to do the same for several models. This what I have now.

There are two models: Company and User.
company has_many :users
user has_one :company

routes.rb
resources :companies
resources :users
resources :companies do
  resources :users
end

Those routes allow me to do this:
companies/2/users/new

Which is good, but once I submit the form, the app seems to forget the
company_id.

I have tried in so many ways and can't get it to work. I did something
yesterday that made the app have an redirect loop (too many redirects)
and it took me over an hour just to fix it. So I'm just going to start
all over again.

My initial question had to do with other models but it's basically the
same. I need to do the same for several models. This what I have now.

There are two models: Company and User.
company has_many :users
user has_one :company

routes.rb
resources :companies
resources :users
resources :companies do
resources :users
end

Those routes allow me to do this:
companies/2/users/new

Which is good, but once I submit the form, the app seems to forget the
company_id.

You have both a regular route and a nested route for your users. Make sure your form is submitting to the nested route.
It should look something like (depending on your variable names):

form_for [@company, @user] do |f|

Without the company your form will post to /users and the company id will not be passed.

Juan Alvarado

You have both a regular route and a nested route for your users. Make
sure your form is submitting to the nested route.
It should look something like (depending on your variable names):

form_for [@company, @user] do |f|

Without the company your form will post to /users and the company id
will not be passed.

I only want to create a new User not a new Company. The Company is
pre-existent. A lot of tutorials I found don't explain how to do that,
they explain how to create the Parent and Child at the same time. I
ALREADY have the Parent, I just want to create a Child for it, and have
the id of the parent written into the Child table, like parent_id.

How would I make the results get written into a user record? Yesterday
when I was trying to work on it, I read something about a "build"
method. Is that what you recommend I use?

You have both a regular route and a nested route for your users. Make
sure your form is submitting to the nested route.
It should look something like (depending on your variable names):

form_for [@company, @user] do |f|

Without the company your form will post to /users and the company id
will not be passed.

I only want to create a new User not a new Company. The Company is
pre-existent.

Using the company in the form_for doesn't mean you are creating a new company as well. That is just generating the nested url to submit the form to. Leaving out the @company will mean that the form get submitted to /users instead of /companies/:company_id/users. Which if I understood you correctly is where you want to submit your form to so that you will have the company id in your params list.

A lot of tutorials I found don't explain how to do that,
they explain how to create the Parent and Child at the same time. I
ALREADY have the Parent, I just want to create a Child for it, and have
the id of the parent written into the Child table, like parent_id.

How would I make the results get written into a user record? Yesterday
when I was trying to work on it, I read something about a "build"
method. Is that what you recommend I use?

You use build on an association to get a new instance of that association.
In your users controller you should have something like:

def create
  @company = Company.find params[:company_id]
  @user = @company.users.build params[:user]
  
  ...
end

Using the build on the users association will automatically set the parent id. It is the same as doing:

@user = User.new params[:user]
@user.company_id = @company_id

and setting any other conditions you have specified on the association manually.

Juan

Not solving the basic problem, but a suggestion for a way of working
that can save a lot of time. Use a Source Control System (git is my
favourite) for your source and commit the code regularly (several
times an hour sometimes, dependant on what you are doing). Then when
you accidentally mess something up and cannot work out what you did it
is easy to go back to previous versions and get it working again. It
may seem like a significant learning curve when you want to get on and
develop your app, but it will save you a huge amount of time in the
long run.

Colin