Data model macro cannot be used in preamble

Question

I'm using this answer to create an annotated bibliography with biblatex, with the following code:

\documentclass{article}

% This just makes a dummy bib file
\begin{filecontents}{\jobname.bib}
@ARTICLE{a,
   author = {Doe, J.},
   title = {The Title},
   journal = {The Journal},
   mynote = {This source is really interesting because it doesn't have a real title}
}

@ARTICLE{b,
   author = {Smith, J.},
   title = {The New Title},
   journal = {The Same Journal},
   mynote = {This second source is also really interesting because it contains words}
}
\end{filecontents}

% This does the work
\usepackage[style=trad-plain]{biblatex}
\addbibresource{\jobname.bib}
\DeclareDatamodelFields[type=field,datatype=literal]{mynote}

\usepackage{xpatch}
\xapptobibmacro{finentry}{\par\printfield{mynote}}{}{}

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

However, I get a warning

Package biblatex Warning: Data model macro 'DeclareDatamodelFields' cannot be used in preamble.

and the output is as if no change had been done to the data model:

Answer 1

Starting from version 2.9 of biblatex (with this commit) data model macros (like \DeclareDatamodelEntrytypes, DeclareDatamodelFields, \DeclareDatamodelEntryfields, ... a full list can be found in §4.5.3 Data Model Specification, pp. 156-161 of the biblatex documentation) are disabled in the preamble and yield only a warning along the lines of

Data model macro '\DeclareDatamodelFields' cannot be used in preamble

These commands can now only be used from an external .dbx (data model) file.

The solution consists in moving these data model macros to an external .dbx file.

In our example we will call said file mynote.dbx and we only have one line to move

\DeclareDatamodelFields[type=field,datatype=literal]{mynote}

We then call this data model via datamodel=mynote in the biblatex options.

MWE

\documentclass{article}

% This just makes a dummy bib file
\begin{filecontents}{\jobname.bib}
@ARTICLE{a,
   author = {Doe, J.},
   title = {The Title},
   journal = {The Journal},
   mynote = {This source is really interesting because it doesn't have a real title}
}

@ARTICLE{b,
   author = {Smith, J.},
   title = {The New Title},
   journal = {The Same Journal},
   mynote = {This second source is also really interesting because it contains words}
}
\end{filecontents}

% This does the work
\usepackage[style=trad-plain,datamodel=mynote]{biblatex}
\addbibresource{\jobname.bib}


\usepackage{xpatch}
\xapptobibmacro{finentry}{\setunit{\par}\printfield{mynote}}{}{}

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

If mynote.dbx is just

\DeclareDatamodelFields[type=field,datatype=literal]{mynote}