Modules in Mercur never reference each other directly. They connect through
module links. The offer sits at the center of the marketplace, so it links
out to the store, the master catalog, pricing, inventory, fulfillment, and the
cart and order line items that reference it. Once a link is defined, you retrieve
related records with query.graph using the link alias.
Catalog & store
Pricing & inventory
Fulfillment
Cart & order
Read-only links (Product, Variant, Seller, Shipping profile) resolve from a
field on the offer row and can’t be written through the link itself. The
Price and Inventory-item links are writable and are managed by the offer
workflows.
The cart- and order-line-item links are list-links on the line-item side so
the same offer can be added to many carts and placed on many orders. Without
that, Medusa would enforce a 1:1 line-item ↔ offer relationship and block
re-use.
