When "Create variations from all attributes" is used to create
variations it generates term relationship entries for all the generated
variations, however that doesn't happen when the term can be interpreted
as a numeric value. This is because in that case product->get_attributes
returns the attribute values as numbers, but the code that generates
the term relationships expect those to always be strings.
When manually adding a given variation this doesn't happen.
The fix is to simply strval-ize the value before using it, but it might
be worth investigating why this is happening.
The previous commit fixes a bug that causes the "attribute_" metadata
with an empty value to not be created when a new variation attribute
is added to the product (so that all variations have the attribute
with a value of "Any..."). This commit adds a data migration to
backfill the missing metadata for existing variations.
When a new variation attribute is added, the corresponding 'attribute_'
meta entries are added for all variations with an empty value;
and when an existing variation attribute is removed, the existing
'attribute_' meta entries are deleted for all variations.
This is necessary for the filter by attribute widget to work properly
when variations exist with a value of "Any..." for attributes.
When a variation product has an attribute with a value of "Any...",
and there's a filter by attribute widget for that attribute, then
that product won't be included in the counts displayed in the widget
(and if the count ends up being zero, the attribute won't be shown
in the widget).
This happens before since Woo 4.4, this widget works by looking at
entries in the term relationships table for varitions too
(used to do so for simple products and for "main" variable products
only), see PR #26260; but there are no such entries for
"Any..." attributes.
This commit fixes that by extending the SQL query used by the widget
to look for variations that have empty attribute values in the meta
table too.