Convert flexible field values to SQL before storing them

[Maxr1998]

Description

This is required to support list values in flexible fields.
See #5698 for more details.

To Do

• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[Copilot]

Pull Request Overview

This PR adds conversion of flexible field values to their SQL representation before inserting them into the database, enabling support for multivalued flexible fields.

• Applies to_sql on each flexible field value prior to storage.
• Removes keys from the dirty set as before, then converts and inserts the SQL value.
• (To be added) Changelog and tests for the new behavior.

Comments suppressed due to low confidence (2)

beets/dbcore/db.py:607

• [nitpick] Overwriting the original value variable may reduce readability; consider assigning the converted value to a new variable (e.g., sql_value) and using that in the mutate call.

                    value = self._type(key).to_sql(value)

beets/dbcore/db.py:607

• Add tests to cover the new to_sql conversion path for flexible fields, verifying that multivalued and edge-case values produce the expected SQL representations.

                    value = self._type(key).to_sql(value)

[Maxr1998]

I'm having a hard time writing tests for this. It looks like my types aren't getting registered:

diff --git a/test/test_plugins.py b/test/test_plugins.py
--- a/test/test_plugins.py	(revision 40d83b5b32b313d8dd7a13a1ed989b3760fdacdd)
+++ b/test/test_plugins.py	(date 1751198533085)
@@ -94,6 +94,17 @@
         out = self.run_with_output("ls", "rating:3..5")
         assert "aaa" not in out
 
+    def test_multi_value_flex_field_type(self):
+        class MultiValuePlugin(plugins.BeetsPlugin):
+            item_types = {"multi_value": types.MULTI_VALUE_DSV}
+
+        self.register_plugin(MultiValuePlugin)
+        assert plugins.types(Item) is not None
+
+        item = Item(path="apath", artist="aaa")
+        item.multi_value = ["one", "two", "three"]
+        item.add(self.lib) # throws an error
+
 
 class ItemWriteTest(PluginLoaderTestCase):
     def setUp(self):

Any ideas?

[Maxr1998]

@snejus, did you have a chance to look into this already?

[snejus]

@snejus, did you have a chance to look into this already?

Yes, I finally have! You've uncovered a fundamental issue with how we register types and queries provided by plugins, which makes it nearly impossible to test properly.

I've just opened a PR with a fix. You can try to rebase on top of it and try testing again :)

[Maxr1998]

Thank you so much! Seems to work now. I'll finish my tests and push everything when it's done.

[snejus]

@Maxr1998 it's been merged into master, so feel free to rebase and add your test 🎉

[Maxr1998]

@snejus thank you so much for fixing it! Just rebased again and pushed my test.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.73%. Comparing base (bde5de4) to head (8db661b).

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