Rails: undefined method text_field_tag

My ERB file works fine if I use text_field, but if I switch to text_field_tag I receive this error:

undefined method `text_field_tag' for #<ActionView::Helpers::FormBuilder:0x00000001f6fd50>

Here is the code that works:

<%= f.text_field mystring %>

And the code that does not work:

<%= f.text_fie开发者_运维知识库ld_tag mystring %>

text_field_tag is documented. How to make it work? Do I need a require or something?

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For your information, text_field_tag is from ActionView::Helpers::FormTagHelper, which states :

Provides a number of methods for creating form tags that doesn’t rely on an Active Record object assigned to the template like FormHelper does. Instead, you provide the names and values manually.

Since this is a helper that does not rely on an active record object, you cannot call this method for the "f" object. It's a a helper method that should be called like this :

<%= text_field_tag "whatever you want to write" %>

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Needed to remove f:

<%= text_field_tag mystring %>

I guess text_field_tag does not rely on the form_for.