Rails 4, check_box_tag and its corresponding HTML

I have a basic question about checkboxes in forms.

I have a simple form for user interests, which has two fields - id and interest_id

    <%= simple_form_for (@user_interest) do |f| %>

    <%= f.error_notification %>

    <%= f.label :interest_id  %><br>
    <%=f.text_field :interest_id, class: 'form-control'%>

    <%=  check_box_tag :interest_id %>

    <%= f.submit "Update my account", class: "btn btn-primary" %>

    <% end %>

If I look at the resulting HTML

 <input class="form-control" type="text" name="user_interest[interest_id]" id="user_interest_interest_id">
 <input type="checkbox" name="interest_id" id="interest_id" value="1">

I am having difficuly understanding why the the name of the text_field (which works) is correct, but the name of the checkbox (which doesn't work) is "interest_id" (and not user_interest[interest_id]

Fully appreciate this is completely basic but actually, these very basic questions can be difficult to get answers to. Many thanks.