biblatex multiple sourcemap : add field to entry

Question

I am trying to add a field to biblatex entries in the texfile. I am currently not able to apply a rule to one entry then apply it other rules to other entries.

I defined a command to add a field. Something is wrong with it but I don't know what it is:

\newcommand{\addnotetoentry}[2]{%
  \DeclareSourcemap{%
  \maps[datatype=bibtex]{%
    \map[overwrite]{%
      \step[fieldsource=entrykey,match=\regexp{^#1$},final]%
      \step[fieldset=note,fieldvalue={#2}]%
    }%
  }%
}%
}

Which is used like this:

\addnotetoentry{a}{one note --- hello my friend}
\addnotetoentry{b}{another note}

MWE:

\documentclass[12pt,letterpaper]{article}
\usepackage[backend=biber,style=alphabetic-verb,doi=false,eprint=false]{biblatex}
\usepackage{filecontents}

\begin{filecontents}{jobname.bib}
@ARTICLE{a,
   author = {Doe, J.},
   title = {The Title},
   journal = {The Journal},
}
@ARTICLE{b,
   author = {Smith, J.},
   title = {The New Title},
   journal = {The Same Journal},
}
\end{filecontents}
\addbibresource{jobname.bib}

\newcommand{\addnotetoentry}[2]{%
  \DeclareSourcemap{%
  \maps[datatype=bibtex]{%
    \map[overwrite]{%
      \step[fieldsource=entrykey,match=\regexp{^#1$},final]%
      \step[fieldset=note,fieldvalue={#2}]%
    }%
  }%
}%
}
\addnotetoentry{a}{one note}
\addnotetoentry{b}{another note}
\AtEveryBibitem{\printfield{note}\clearfield{note}\item}

\begin{document}

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

I get in the ouput's references only one note (the last one that I defined):

Answer 1

The problem with Biber's \DeclareSourcemap is that it may only be used once in a document (only the last instance will be executed, all the other calls will be ignored)*, as such it is quite hard to implement a macro using \DeclareSourcemap.

If you just want to add a comment to some bibliography entries, we can go down a different route.

\newcommand*{\writecomment}[2]{\csdef{localcomment#1}{#2}}

We define \writecomment to define a macro localcomment<entrykey> (<entrykey> as given in #1) that takes the value #2.

That macro localcomment<entrykey> can then be used as \csuse{localcomment<entrykey>}, like so

Before every element:

\AtEveryBibitem{\csuse{localcomment\thefield{entrykey}}\item}

Or after every element:

\renewbibmacro*{finentry}{%
  \finentry
  \ifcsdef{localcomment\thefield{entrykey}}
    {\\\csuse{localcomment\thefield{entrykey}}}
    {}%
}

\csuse will not do anything if its argument is not a valid command sequence; that means, if the macro was not created via \writecomment this will result in no output at all.

Another plus side of this solution is that we do not have to fiddle with the note field and do not override existing notes.

Usage of \writecomment is straightforward: \writecomment{<entrykey>}{<note>}

\writecomment{a}{first comment}
\writecomment{b}{second comment}

MWE

\documentclass{article}
\usepackage[backend=biber,style=alphabetic-verb,doi=false,eprint=false]{biblatex}
\begin{filecontents}{jobname.bib}
@ARTICLE{a,
   author = {Doe, J.},
   title = {The Title},
   journal = {The Journal},
}
@ARTICLE{b,
   author = {Smith, J.},
   title = {The New Title},
   journal = {The Same Journal},
}
@ARTICLE{c,
   author = {Miller, J.},
   title = {The New Title},
   journal = {Won't Get A Comment},
}
\end{filecontents}
\addbibresource{jobname.bib}

\newcommand*{\writecomment}[2]{\csdef{localcomment#1}{#2}}
\AtEveryBibitem{\csuse{localcomment\thefield{entrykey}}\item}

\writecomment{a}{first comment}
\writecomment{b}{second comment}

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

* I cannot find any piece of documentation that backs this claim up, but I'm quite confident it is correct.

Answer 2

So apparently you cannot add multiple \DeclareSourcemap to a latex file since only the last one it taken into account.

---

One Solution can be to declare only one source map and modify the entries manually

\DeclareSourcemap{
  \maps[datatype=bibtex]{
    \map[overwrite]{
      \step[fieldsource=entrykey,match=\regexp{^a$},final]
      \step[fieldset=note,fieldvalue={one note}]
    }
    \map[overwrite]{
      \step[fieldsource=entrykey,match=\regexp{^b$},final]
      \step[fieldset=note,fieldvalue={another note}]
    }
  }
}