Automatically prepend "http://" in bibliographic url hyperlinks?

Question

I use references that have an address in the url field. They use following template in my .bib file: ssrn.com/abstract=1234567.

However, the generated hyperlink is also ssrn.com/abstract=1234567. This makes pdf readers not want to open the link, since there is no http:// prefix. Adding the latter for each URL would (be tedious, and) take too much place in the list of references.

Question: How to make biblatex generate an hyperlink like \href{http://<url-field-content>}{<url-field-content>} in the references?
(where <url-field-content> is information given by the .bib file)

---

\documentclass{article}

    \usepackage[style=authoryear-comp]{biblatex}
        \addbibresource{\jobname.bib}
        \begin{filecontents}{\jobname.bib}
            @book{key,
                author = {Author, A.},
                year = {2001},
                title = {Title},
                publisher = {Publisher},
                url = {ssrn.com/abstract=1234567},
            }
        \end{filecontents}

    \usepackage{filecontents}
    \usepackage{hyperref}

\begin{document}
    \nocite{key}
    \printbibliography
\end{document}

Answer 1

You can do this by redefining the url field.

% arara: pdflatex
% arara: biber
% arara: pdflatex
% arara: pdflatex
\begin{filecontents}{\jobname.bib}
  @book{key,
    author = {Author, A.},
    year = {2001},
    title = {Title},
    publisher = {Publisher},
    url = {ssrn.com/abstract=1234567},
  }
\end{filecontents}

\documentclass{article}

\usepackage[style=authoryear-comp]{biblatex}
\DeclareFieldFormat{url}{%
  \mkbibacro{URL}\addcolon\space
  \href{http://#1}{\UrlFont#1}%
}
\addbibresource{\jobname.bib}

\usepackage{hyperref}

\begin{document}
\nocite{key}
\printbibliography
\end{document}

Answer 2

Use Biber's sourcemapping. This allows you to not modify URL fields that are already correct. The following adds http:// to url fields that do not start with ftp://, http:// or https://.

\documentclass{article}
\usepackage[style=authoryear-comp]{biblatex}

\usepackage{filecontents}
\begin{filecontents}{\jobname.bib}
@online{bronto,
  author   = {Anne Elk},
  title    = {Towards a Unified Theory on Brontosauruses},
  date     = {1972-11-16},
  url      = {example.edu/~elk/bronto.pdf},
  urldate  = {2015-09-07},
}
@online{tric,
  title    = {A Theory on Triceratops},
  author   = {Anne Elk},
  date     = {1972-11-16},
  url      = {https://example.edu/~elk/tric.html#page11},
  urldate  = {2015-09-07},
}
\end{filecontents}
\addbibresource{\jobname.bib}

\usepackage{hyperref}

\DeclareSourcemap{
  \maps[datatype=bibtex]{
    \map{
      \step[fieldsource=url, notmatch=\regexp{\A(ftp|https?)://}, final]
      \step[fieldsource=url, match=\regexp{\A(.*)\Z}, replace=\regexp{http://$1}]
    }
  }
}

\begin{document}
    \nocite{*}
    \printbibliography
\end{document}