Convert flexible field values to SQL before storing them (#5833)

## Description

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

beets/dbcore/db.py

@@ -605,6 +605,7 @@ class Model(ABC, Generic[D]):
             for key, value in self._values_flex.items():
                 if key in self._dirty:
                     self._dirty.remove(key)
+                    value = self._type(key).to_sql(value)
                     tx.mutate(
                         "INSERT INTO {} "
                         "(entity_id, key, value) "

docs/changelog.rst

@@ -101,6 +101,10 @@ For plugin developers:
 - ``plugins.load_plugins`` function does not anymore accept the list of plugins
   to load. Instead, it loads all plugins that are configured by
   :ref:`plugins-config` configuration.
+- Flexible fields, which can be used by plugins to store additional metadata,
+  now also support list values. Previously, beets would throw an error while
+  storing the data in the SQL database due to missing type conversion.
+  :bug:`5698`
 
 Other changes:
 

test/test_plugins.py

@@ -46,7 +46,10 @@ from beets.util import displayable_path, syspath
 
 class TestPluginRegistration(PluginTestCase):
     class RatingPlugin(plugins.BeetsPlugin):
-        item_types = {"rating": types.Float()}
+        item_types = {
+            "rating": types.Float(),
+            "multi_value": types.MULTI_VALUE_DSV,
+        }
 
         def __init__(self):
             super().__init__()
@@ -83,6 +86,15 @@ class TestPluginRegistration(PluginTestCase):
 
         assert MediaFile(syspath(item.path)).artist == "YYY"
 
+    def test_multi_value_flex_field_type(self):
+        item = Item(path="apath", artist="aaa")
+        item.multi_value = ["one", "two", "three"]
+        item.add(self.lib)
+
+        out = self.run_with_output("ls", "-f", "$multi_value")
+        delimiter = types.MULTI_VALUE_DSV.delimiter
+        assert out == f"one{delimiter}two{delimiter}three\n"
+
 
 class PluginImportTestCase(ImportHelper, PluginTestCase):
     def setUp(self):