how to compile subfiles with bibtex citations?

Question

Consider the following main.tex:

\documentclass{report}
\usepackage{filecontents}
\usepackage{subfiles}
\begin{filecontents}{\jobname.bib}
@book{key,
  author = {Author, A.},
  year = {2001},
  title = {Title},
  publisher = {Publisher},
}
\end{filecontents}

\begin{document}
\subfile{chapter1.tex}
\bibliographystyle{unsrt}
\bibliography{\jobname}
\end{document} 

Here is chapter1.tex:

\documentclass[main.tex]{subfiles}
\begin{document}
\chapter{My Title}
\cite{key}   % the problem
\end{document}

Compiling main.tex with pdflatex bibtex pdflatex pdflatex and all is good. But a primary goal of subfiles is to allow subfile compilation, and I cannot figure out how to compile chapter1.tex in this way with the citation defined.

Can someone please post the exact changes that will allow this (with bibtex)? The "hints" in the subfiles documentation are not enough for me. (I tried \subfix without luck.)

Answer 1

If we run bibtex chapter1 (after having run pdflatex chapter1) we get

This is BibTeX, Version 0.99d (MiKTeX 2.9.7380 64-bit)
The top-level auxiliary file: chapter1.aux
I found no \bibdata command---while reading file chapter1.aux
I found no \bibstyle command---while reading file chapter1.aux

so BibTeX complains about not knowing which style to take and which .bib file to access. Essentially that is because chapter1.tex has neither a \bibliographystyle call to set the bibliography style, nor a \bibliography call to tell BibTeX which .bib file to use. The subfile can read the preamble of its parent, but not the document body, so it never gets to see the \bibliographystyle{unsrt} and \bibliography{\jobname} in main.tex.

We need a way to tell BibTeX about the style and the .bib file.

For the bibliography style that is simple, because \bibliographystyle may be used in the preamble, so you could just make your main.tex read

\documentclass{report}

\usepackage{subfiles}

\bibliographystyle{unsrt}

\begin{document}
\subfile{chapter1.tex}

\bibliography{mybibfile}
\end{document}

But that still leaves chapter1.tex unaware of \bibliography. Unfortunately, it is not possible to simply move \bibliography into the preamble, because (with BibTeX) \bibliography must be used at the place where the bibliography is printed.

You need a \bibliography call in chapter1.tex if you want to compile the file separately with working citations. But just adding \bibliography{mybibfile} to chapter1.tex will lead to two bibliographies (and/or an error) in main.tex. Ideally, we'd find a way to check if we are compiling the subfile chapter1.tex on its own or in the context of the full file main.tex. I didn't find an official check for that in the documentation, but think it would be a useful addition, so I had to roll my own test that replicates \@ifclassloaded: If chapter1.tex is compiled on its own, the class will be subfiles, if it is compiled in the context of main.tex the class is different.

So here we are: \subfilesbibliography creates a bibliography when the file is compiled on its own, but not when it is compiled in the main document.

main.tex

\documentclass{report}

\usepackage{subfiles}

\bibliographystyle{unsrt}

\makeatletter
% curse you, \@onlypreamble\@ifclassloaded
\newcommand*{\subfilesbibliography}[1]{%
 \expandafter\ifx\csname ver@subfiles.cls\endcsname\relax
   \expandafter\@secondoftwo
 \else
   \expandafter\@firstoftwo
  \fi
  {\bibliography{#1}}
  {}%
}
\makeatother

\begin{document}
\subfile{chapter1.tex}

\bibliography{mybibfile}
\end{document}

chapter1.tex

\documentclass[main.tex]{subfiles}
\begin{document}
\chapter{My Title}
\cite{key}   % the problem

\subfilesbibliography{mybibfile}
\end{document}

Note that I used a fixed file name instead of \jobname, because the variable nature of \jobname would make things much more complicated.