Metadata definition for a user-defined field

[LARdT]

I want to define a metadata field in content.opf in such a way that when I press the "read metadata from book" option the metadata content will be inserted into a user-defined field called "#version_css".

How must I define the metadata field in content.opf?

[Manichean]

You could try going the opposite way: Create a custom column #version_css, add some value to a book, wait for Calibre to create the .opf file. Then, dissect the file to find out how to do it.

Edit to add: I don't know, however, if Calibre even tries to read user defined metadata fields while importing.

[LARdT]

Well, I've tried that already. And I don't seem to be able to trigger the metadata writing-back to the file.
Could you please try it yourself and detail the procedure?

Thanks

[Manichean]

For the custom column in my library, I'm seeing the following entry in the OPF:

        <meta name="calibre:user_metadata:#fach" content="{&quot;is_category&quot;: false, &quot;colnum&quot;: 2, &quot;kind&quot;: &quot;field&quot;, &quot;is_custom&quot;: true, &quot;name&quot;: &quot;Fachbuch&quot;, &quot;#extra#&quot;: null, &quot;datatype&quot;: &quot;bool&quot;, &quot;rec_index&quot;: 20, &quot;search_terms&quot;: [&quot;#fach&quot;], &quot;#value#&quot;: true, &quot;link_column&quot;: &quot;value&quot;, &quot;label&quot;: &quot;fach&quot;, &quot;is_editable&quot;: true, &quot;column&quot;: &quot;value&quot;, &quot;is_multiple&quot;: null, &quot;category_sort&quot;: &quot;value&quot;, &quot;table&quot;: &quot;custom_column_2&quot;, &quot;display&quot;: {}}"/>

This is a tristate binary column. From that, I'm guessing that user defined columns are stored in metadata entries of the form

<meta name="calibre:user_metadata:#name_here" content="settings_here"/>

I'm also seeing that extracting the settings would take quite a bit of matching.

[kovidgoyal]

click the arrow next to the calibre icon and choose library maintenance->start metadata backup to cause a refresh of all opf files (I'd suggest creating a small custom library to play with it).