Full cite of cross-referenced entry

Question

I have an entry key foo in my (biblatex-biber managed) bibliography, with a crossref field pointing to another entry key. To automate type-setting operations on a series of files, I would like to be able to cite the cross-referenced entry with something like: \citefull{\citefield{foo}{crossref}}

This naive syntax does not work, as biblatex commands are not nestable/expandable. Any hints on how to proceed?

Bonus, that could be handy too: \citefull[\citefield{foo}{note}]{\citefield{foo}{crossref}}

What I've tried: Many posts mention this nesting/expanding limitation and propose a workaround. I thus know this is somehow possible, but I don't understand the codes well enough to adapt them to my needs. For example this question seems quit closely related. The closest that I got was tweaking this answer, which is supposed to be more complex that what I need, as it deals with name list and I only want to expand a monolithic field. My tweak below does print the full reference of key num3 when calling key num4 (whose author field contains num3) like I want it to. However, I was not able to call the crossref field (nor a custom verba field) instead of the author field.

\documentclass{article}                                                      
\usepackage[american]{babel}                                                 
\usepackage[backend=biber,firstinits]{biblatex}                              

\usepackage{filecontents}                                                    
\begin{filecontents}{\jobname.bib}                                           
@misc{num03,                                                                 
title = {Title XX3},                                                         
author = {Author2 Authorsen2 and Author2a Authorsen2a},                      
}                                                                            
@misc{num04,                                                                 
title = {Title},                                                             
author = {num03},                                                            
verba = {num03},                                                             
crossref = {num03},                                                          
}                                                                            
\end{filecontents}                                                           
\addbibresource{\jobname.bib}                                                

\newcommand*{\name}{}                                                        
\DeclareIndexNameFormat{getname}{%                                           
\nameparts{#1}                                                               
\ifblank{\namepartgiveni}                                                    
  {}                                                                         
  {\iffirstinits                                                             
     {\xappto{\name}{\namepartgiveni\space}}                                 
     {\xappto{\name}{\namepartgiven\space}}}%                                
\ifblank{\namepartprefix}                                                    
  {\xappto{\name}{\namepartfamily}}                                          
  {\xappto{\name}{\namepartprefix\space\namepartfamily}}}                    
\DeclareCiteCommand{\getauthor}                                              
  {\undef\name                                                               
   \boolfalse{citetracker}%                                                  
   \boolfalse{pagetracker}}                                                  
  {\ifnameundef{author}                                                      
     {}                                                                      
     {\ifboolexpr{ test {\ifnumgreater{\value{citecount}}{1}}                
                   and not test {\ifundef{\name}} }                          
        {\gappto{\name}{; }}                                                 
        {}%                                                                  
      \indexnames[getname][1-99]{author}}}                                   
  {}                                                                         
  {}                                                                         

\begin{document}                                                             
\getauthor{num04}                                                            
\fullcite{\name}                                                             
\end{document}                                                               

Which, after the correct pdflatex/biber runs, gives:

I also have another (ugly) workaround based on an execute field containing a \nocite{num03}. I then tweak printbilbiography and it kind of goes the way I need... But it really doesn't sound like a proper way of doing this!

Answer 1

Unfortunately, the way entry data is saved in biblatex means we can't expandably get hold of field values (which is a bit of a bummer), so we will have to work with helper macros that hold the field value.

\makeatletter
% {<command>}{<entrykey>}{<field>}
\newcommand*{\getfieldinto}[3]{%
  \nocite{#2}%
  \begingroup
  \blx@getdata@cite{#2}%
  \edef\my@tempa{\endgroup
    \def\unexpanded{#1}{%
      \blx@imc@iffieldundef{#3}
        {}
        {\csexpandonce{abx@field@#3}}}}%
  \my@tempa}
\makeatother

Works similar to the code in the question that extracts the name expandably, but works on a field level and is less complicated, because it does not have to deal with the complexities of name handling in biblatex.

\getfieldinto{<command>}{<entrykey>}{<field>}

extracts the contents of <field> for <entrykey> into the macro <command>. So you would use it like

\getfieldinto{\mycomm}{num04}{verba}

to get num04's verba into \mycomm.

The fields crossref and xref that one might intuitive want to use here can't be used because they are special and may not actually make it to the .bbl file (the entry data read by biblatex), so something like verba (or a custom verb field) is probably the safest option.

\documentclass{article}
\usepackage[american]{babel}
\usepackage{csquotes}
\usepackage[backend=biber, giveninits]{biblatex}

\makeatletter
% {<command>}{<entrykey>}{<field>}
\newcommand*{\getfieldinto}[3]{%
  \nocite{#2}%
  \begingroup
  \blx@getdata@cite{#2}%
  \edef\my@tempa{\endgroup
    \def\unexpanded{#1}{%
      \blx@imc@iffieldundef{#3}
        {}
        {\csexpandonce{abx@field@#3}}}}%
  \my@tempa}
\makeatother

\begin{filecontents}{\jobname.bib}
@misc{num03,
  title  = {Title XX3},
  author = {Author2 Authorsen2 and Author2a Authorsen2a},
}
@misc{num04,
  title    = {Title},
  verba    = {num03},
  note     = {note},
}
\end{filecontents}
\addbibresource{\jobname.bib}

\begin{document}
\getfieldinto{\mykey}{num04}{verba}
\getfieldinto{\mynote}{num04}{note}
\fullcite[\mynote]{\mykey}
\end{document}

It is possible to access the entry data of a different entry from within a cite command, so there may well be a way to solve your problem differently (and possibly more elegantly), but for that we would have to know what your actual use case is about.