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I have recently started converting some of my non-epub documents to the epub format. I see in Calibre that there is a 'Tags' column that contains genera information (screenshot below).

I would like to add this tag data directly to my epub document so it can be immediately used by Calibre without needing to go to the internet to download additional metadata (this is super slow for me). However, I have been unable to locate if or how this data is saved into the epub itself.

I have extracted other epubs that have the tag column populated in Calibre and analyzed the ncx and opf files but I haven't come across any property that contains this information.

Does this mean that the data in the 'Tags' column is stored internally to the Calibre database?

If I created a custom '<tags>' property in the opf file, is there a way I could get it to import automatically into Calibre?

Thanks in advance for any and all support!

enter image description here

Update: I have learned that when you set properties on an ebook in calibre you can write these new attributes into the ebook using the 'embed metadata' function. After writing the properties I can see the following values get set in the OPF file:

  • Rating (Calibre property):
<opf:meta property="calibre:rating">8</opf:meta>
  • Tags (Calibre property):
<dc:subject>cali_tags_1</dc:subject>
<dc:subject>cali_tags_2</dc:subject>
<dc:subject>cali_tags_1.cali_tags_1_nested_1</dc:subject>
  • Custom properties(Review,description):
<opf:meta property="calibre:user_metadata">{
 "#description": {
   "#extra#": null,
   "#value#": "Cali Created Desc",
   "category_sort": "value",
   "colnum": 2,
   "column": "value",
   "datatype": "comments",
   "display": {
     "description": "",
     "heading_position": "hide",
     "interpret_as": "long-text"
   },
   "is_category": false,
   "is_csp": false,
   "is_custom": true,
   "is_editable": true,
   "is_multiple": null,
   "is_multiple2": {},
   "kind": "field",
   "label": "description",
   "link_column": "value",
   "name": "Description",
   "rec_index": 22,
   "search_terms": [
     "#description"
   ],
   "table": "custom_column_2"
 },
 "#rating": {
   "#extra#": null,
   "#value#": 9.66,
   "category_sort": "value",
   "colnum": 3,
   "column": "value",
   "datatype": "float",
   "display": {
     "decimals": 2,
     "description": "",
     "number_format": null
   },
   "is_category": false,
   "is_csp": false,
   "is_custom": true,
   "is_editable": true,
   "is_multiple": null,
   "is_multiple2": {},
   "kind": "field",
   "label": "rating",
   "link_column": "value",
   "name": "Review",
   "rec_index": 23,
   "search_terms": [
     "#rating"
   ],
   "table": "custom_column_3"
 }
}</opf:meta>

If I re-add a book where I've set the properties in Calibre, all of the attributes are detected successfully. However, if I set these same properties using a script, they aren't detected. The nesting of the properties is the same. So why aren't the custom properties I set being detected? Does order matter?

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