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In this document, you’ll learn how an attribute becomes the axis a product’s variants are generated from, and the mirror links that keep the two in sync.

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Variant-axis attribute

A multi_select attribute marked is_variant_axis is more than a descriptor. It defines a dimension along which a product varies, such as Size or Color. When such an attribute is created, the ProductAttribute model records the id of a native Medusa ProductOption in its product_option_id field, and each of its ProductAttributeValues records the matching ProductOptionValue id in product_option_value_id.
Because the attribute mirrors a real ProductOption, the values a product selects along that axis are exactly what Medusa uses to generate its variants. The attribute catalog and Medusa’s product options are two separate modules, so the relationship is kept as a pair of read-only mirror links: Both are 1:1 and have no pivot table. The foreign key lives on the attribute record itself.
The mirror links are read-only. They’re resolved from the FK on the attribute record; you never write the relationship through the link. The workflows keep the option and the attribute in step whenever an axis attribute or its values change.
Only multi_select attributes can be variant axes. The other four types (single_select, text, unit, toggle) describe a product but never generate variants.