Biblatex: create a bib entry automatically from \addCitation{http://myurl}

Question

In a document, I need to add many links via a biblatex entry with something like:

\NewDocumentCommand{\addLink}{O{}mm}{\href{#2}{#3}}
\addLink{https://en.wikipedia.org/}{Wikipedia}\cite{Wiki}

where the bib contains:

@online{Wiki,
    title        = {Wikipedia},
    author       = {Wikipedia},
    year         = 2024,
    url          = {https://en.wikipedia.org/},
}

But manually maintaining these two lists is quite expensive, and adding a bib entry for a simple link is quite annoying and hard to maintain. Would it be possible, somehow, to avoid this by creating automatically the entry in the \NewDocumentCommand? The year can be fixed to 2024 and the author could be equal to the title unless specified otherwise in the optional argument.

MWE:

\documentclass[]{article}
\begin{filecontents}[noheader,overwrite]{main.bib}
@online{Wiki,
    title        = {Wikipedia},
    author       = {Wikipedia},
    year         = 2024,
    url          = {https://en.wikipedia.org/},
}
\end{filecontents}
\usepackage[
  style=alphabetic,% also 
  minalphanames=3,maxalphanames=4, % [Foo+99] -> [FBB+99].
  maxnames=99, % Do not put "et al". Sets maxbibnames, maxcitenames and maxsortnames.
  sortcites=true,
  %sorting=none,
  doi=false,
  url=false,
  giveninits=true, % Bob Foo --> B. Foo
  isbn=false,
  url=false,
  eprint=false,
  sortcites=false, % \cite{B,A,C}: [A,B,C] --> [B,A,C]
%backref=true, % [1] Title, blabla --> [1] Title, blabla (pages 1, 45, 56)
%% TODO: customize using https://tex.stackexchange.com/questions/36307/formatting-back-references-in-bibliography
]{biblatex}
\addbibresource{main.bib}%
\NewDocumentCommand{\addLink}{O{}mm}{
  \href{#2}{#3}
}
\usepackage{hyperref}
\begin{document}
You can go to \addLink{https://en.wikipedia.org/}{Wikipedia}\cite{Wiki}.
\printbibliography
\end{document}

EDIT I guess I can think of a solution that writes to a temporary file all entries, and that (maybe rename?) and loads this file at startup… but seems like quite a quite dirty solution. Is there a better one?

Answer 1

I don't think there is a fair chance of "injecting" the entry into biblatex's internals purely on the LaTeX side of things, not least because sorting has to be done by Biber.

So I think your best chance is to write to a temporary .bib file and \addbibresource that file. This can all be hidden from the user in helper macros as shown in my answer to Make specific author bold using biblatex.

For example you could try

\documentclass[]{article}
\usepackage[
  style=alphabetic,% also 
  minalphanames=3,maxalphanames=4, % [Foo+99] -> [FBB+99].
  maxnames=99, % Do not put "et al". Sets maxbibnames, maxcitenames and maxsortnames.
  sortcites=true,
  %sorting=none,
  doi=false,
  url=false,
  giveninits=true, % Bob Foo --> B. Foo
  isbn=false,
  url=false,
  eprint=false,
  sortcites=false, % \cite{B,A,C}: [A,B,C] --> [B,A,C]
]{biblatex}
\usepackage{hyperref}
\makeatletter
\def\tbblx@bibfile@name{\jobname -tblx.bib}
\newwrite\tbblx@bibfile
\immediate\openout\tbblx@bibfile=\tbblx@bibfile@name
\immediate\write\tbblx@bibfile{%
  @comment{Auto-generated file}\blx@nl}
\newcounter{tbblx@name}
\setcounter{tbblx@name}{0}
\newcommand*{\tbblx@citeandwritetobib}[3]{%
  \stepcounter{tbblx@name}%
  \edef\tbblx@tmp@cite{%
    \noexpand\autocite{tbblx@name@\the\value{tbblx@name}}}%
  \tbblx@tmp@cite
  \immediate\write\tbblx@bibfile{%
    @online{tbblx@name@\the\value{tbblx@name},
          author  = {\unexpanded{#1}}, %
          title   = {\unexpanded{#2}},
          year    = {2024},
          url     = {\unexpanded{#3}},}%
  }%
}
\AtEndDocument{%
  \closeout\tbblx@bibfile}
\addbibresource{\tbblx@bibfile@name}
\NewDocumentCommand{\addLink}{O{#3}mm}{%
  \href{#2}{#3}
  \tbblx@citeandwritetobib{#1}{#3}{#2}%
}
\makeatother
\addbibresource{biblatex-examples.bib}
\begin{document}
You can go to \addLink{https://en.wikipedia.org/}{Wikipedia}.
\cite{sigfridsson}
\printbibliography
\end{document}

This generates a helper file <jobname>-tblx.bib to which we write all .bib entries on the fly (via \tbblx@citeandwritetobib). In this example all entries are given entry key tbblx@name@<counter>, but you could of course change the code so that you could supply a useful entry key for later re-use. (At the moment you can't easily retrieve the entry key for a specific entry, because the specification said nothing about the entry key you want to use and arbitrary text like author, title or url are unsafe as and shouldn't be used for the entry key. That's also why at the moment \tbblx@citeandwritetobib issues an \autocite to the entry. If you make the entry key predictable, you can separate that out.) Apart from that we just need the usual bookkeeping of opening and closing the output file. (We need to close \AtEndDocument to ensure we can write at any time in the document.)

Answer 2

Just to add to the answer of moewe, I realized that just using the counter was quite error prone (e.g. if I just remove one element, all entries will be shifted without any error if I forget to re-run biblatex).

So I'm using instead:

\usepackage{pdftexcmds}
\makeatletter
\def\tbblx@bibfile@name{\jobname -tblx.bib}
\newwrite\tbblx@bibfile
\immediate\openout\tbblx@bibfile=\tbblx@bibfile@name
\immediate\write\tbblx@bibfile{%
  @comment{Auto-generated file}\blx@nl}
\usepackage{pdftexcmds}
\newcommand*{\tbblx@citeandwritetobib}[5]{%
  \edef\tbblx@name@entry{\pdf@mdfivesum{\detokenize{#1#2#3#4#5}}}%just the counter is not enough, as we might forget to run latexmk to regenerate the entries if one changes. Also, not adding the counter is better as it allows exactly identical entries to share the same line in the bibliography.
  \edef\tbblx@tmp@cite{%
    \noexpand\cite{tbblx@name@\tbblx@name@entry}}%
  \tbblx@tmp@cite
  \immediate\write\tbblx@bibfile{%
    @online{tbblx@name@\tbblx@name@entry,
          author  = {\unexpanded{#1}}, %
          title   = {\unexpanded{#2}},
          year    = {#4},
          url     = {\unexpanded{#3}}
        }%
  }%
}
\AtEndDocument{%
  \closeout\tbblx@bibfile}
\addbibresource{\tbblx@bibfile@name}
\NewDocumentCommand{\mylinkCite}{O{{#3}}mO{#4}mO{2024}}{%
  \href{#2}{#4}
  \tbblx@citeandwritetobib{#1}{#3}{#2}{#5}{}%
}
\makeatother

And I use it like: \mylinkCite[author, default to text]{link}[title, default to text]{text to write in pdf}[year (default 2024)]

Note also that biblatex does not plan to support this feature out of the box as it is too complicated to implement internally Biblatex: create a bib entry automatically from \addCitation{http://myurl}