Radio buttons are not mutually exclusive in Edit page

Rails
1

I have two emails for a contact. The first email is primary.While saving
a new contact I can only set one radio button for primary, but in edit
page the primary radio button can be set for both emails(not mutually
exclusive)

View

<% for email in @contact.emails %> <% fields_for "contact[email_attributes][]", email do |e| %> Email <%= e.select ('email_type',%w{Business Personal}, :include_blank => false) %> <%= e.text_field :email,:size=>"35",:maxlength=>"80" %> <%= e.radio_button ('isprimary', 'true') %>Primary <% end %> <% end %>