Biblatex clear URL field for entries in a certain category

Question

Can you please help me to process biblatex entries. I want to...

1. Print separate bibliographies for online and offline entries
2. Both @online entries and @misc entries with a url-field should go into the 'online' category. All other entries should go into the 'offline' category.
3. I want don't want to display the url and urldate of offline entries.

I have tried combining answers from the following webpages:

• url field for certain reference types with biblatex
• separating two types of articles from bibtex using \printbibliography

When should I use \AtEveryCitekey and when should I use \AtEveryBibitem? Can I issue multiple \AtEvery*** commands, or will one command replace the other?

Here is my minimal (not fully) working example. I can always either get the categorization or the clearing of fields to work, but not both:

\documentclass{article}
\usepackage{biblatex}

\DeclareBibliographyCategory{online}
\DeclareBibliographyCategory{offline}
\AtEveryCitekey{%
    \ifboolexpr{% 
        test {\ifentrytype{online}}
        or
        ( test {\ifentrytype{misc}}
          and not test {\iffieldundef{url}}
        )
    }
    { \addtocategory{online}{\thefield{entrykey}}\clearfield{url}\clearfield{urldate} }
    { \addtocategory{offline}{\thefield{entrykey}} }
}


\usepackage{filecontents}

\begin{filecontents}{\jobname.bib}
@article{c1,
      author         = "Authors1",
      title          = "{Title of article}",
      collaboration  = "Collaboration",
      year           = "2012",
      url            = "http://www.sciencedirect.com/",
      urldate        = "2012-07-12",
}
@online{c2,
      author         = "Authors2",
      title          = "{Title of website}",
      year           = "2011",
      url            = "http://www.gmx.at/",
      urldate        = "2012-06-10",
}
@misc{c3,
      author         = "Authors3",
      title          = "{Title of video}",
      year           = "2012",
      howpublished   = "Youtube video",
      url            = "http://youtube.com/watch/123",
      urldate        = "2012-06-06",      
}
\end{filecontents}

\addbibresource{\jobname.bib}

\begin{document}

\cite{c1,c2,c3}

\printbibliography[category=offline,title={Offline Articles}]

\printbibliography[category=online,title={Online Articles}]

\end{document}

Answer 1

This is best done using biber's sequential source mapping feature which allows you to modify the data as a stream as it's processed, without actually changing the source files:

\documentclass{article}
\usepackage{biblatex}
\addbibresource{test.bib}

\DeclareSourcemap{
  \maps[datatype=bibtex, overwrite=true]{
    \map{
      \step[fieldset=keywords, fieldvalue=offline]
    }
    \map{
      \pertype{online}
      \step[fieldset=keywords, fieldvalue=online]
    }
    \map{
      \pertype{misc}
      \step[fieldsource=url, final]
      \step[fieldset=keywords, fieldvalue=online]
    }
    \map{
      \step[fieldsource=keywords, match=offline, final]
      \step[fieldset=url, null]
    }
  }
}

\begin{document}
\cite{c1,c2,c3}
\printbibliography[keyword=offline,title={Offline Articles}]
\printbibliography[keyword=online,title={Online Articles}]
\end{document}

• First set a default keyword of "offline" for an entry
• Then overwrite this with "online" if the entry is @online or @misc with URL field
• Then delete the URL field if the keywords field is still "offline"
• print two bibliographies conditionalised on the keywords