text_field disabled in Ruby on Rails

Assuming that is_admin? returns true or false, you can simply do

<%= f.text_field :nome, class: "form-control", disabled: !is_admin? %>

Currently, the if condition i.e., if not is_admin? is applied on the entire text field, which results in text field disappearance when is_admin? returns true and when the condition returns false text field is displayed.

The specific reason that your code wasn't working as expected is an operator precedence issue - the if test is being applied to the whole text field, not just the disabled parameter.

The most direct solution to this (though not necessarily the best) is to wrap true if not is_admin? in parentheses:

<%= f.text_field :nome, class: "form-control", disabled: (true if not is_admin?) %>

Otherwise, the whole text field has the if applied to it, like this:

<%= (f.text_field :nome, class: "form-control", disabled: true) if not is_admin? %>

So it will be rendered only for non-admin users - i.e. when is_admin? is false.

Also, when it's rendered, it will always be disabled. (Which on the plus side, will make it harder for non-admins to abuse.)