Using Sinatra with Activerecord Associations

I couldn't find any good tutorial on how to use Activerecord associations with Sinatra. So, I managed to create on in my own style but i am not sure if this is the correct approach. Its working as expected but would appreciate your suggestions. I have two classes Visitors and Users. A visitor can have many users and the visitor can add new users after logging in. Here are the code snippets from the files:

new.erb:

<div class="newuser">
<form id="user" action="/" method="POST" role="form" >
    <div class="form-group">
        <label for="email">Email Address:</label>
        <input type="email" name ="email" value="<%= @user.email %>" placeholder="Email" class="form-control">
    </div>
    <div class="form-group">
        <label for="email">Name:</label>
        <input type="text" name="fname" value="<%= @user.fname %>" placeholder="Name" class="form-control">
    </div>
    <input type = "hidden" value ="<%= session[:id] %>" name="visitor_id" >
    <input type="submit" value="Submit &rarr;" class="btn btn-info">


</form>

</div>

The session id is defined as follows:

@visitor = Visitor.find_by_usernameemail(params[:usernameemail])
session[:id] = @visitor.id

Now my models.rb file is as follows:

class User < ActiveRecord::Base
    validates :fname, presence: true
    validates :email, presence: true
    belongs_to :visitor
end

class Visitor < ActiveRecord::Base
    validates :usernameemail, presence: true
    validates :password, presence: true
    has_many :users
end

the User Id route is written as follows:

get '/user/:id' do
    if User.exists?(params[:id])
        @user = User.find(params[:id])
        if @user.visitor_id == session[:id]
            erb :user
        else
            "Your are not allowed to access this page"
        end
    else
        "record not found"
    end
end

Now using the above approach, is it really necessary to use has_many and belongs_to and is this the proper way of doing it? Also, it would be great if you could point me to a resource on how to use has_many and belongs_to like associations in Sinatra with activerecord.