Getting user ID when calling an action

I have a table for doing CRUD operations on books, and also users can check in books (the checking out is done my a different controller)

At the moment the check in works for the book table, but I can't update user data - the integer of how many books they have loaned out

Here is part of my table:

    <tr><% @books.each do |book| %>

        <td><%= book.title %></td>
        <td><%= book.author %></td>
        <td class ="onloan"><%= book.onloan %></td>
        <td><%= link_to 'show', book %> </td>
        <td><%= link_to 'edit', edit_book_path(book) %></td>
        <td><%= link_to 'Destroy', book, method: :delete, data: { confirm: 'Are you sure?' } %></td>
        <td><%=  link_to 'Check in', check_in_path(book)%></td>

    </tr>

    <% end %>

What I want to add is something like

@user = User.find(params[:id])
@user.books_on_loan - 1

Here is the check in def of my controller:

def check_in


 Book.find(params[:id]).update_attribute(:onloan, 0)

 redirect_to root_path


end