Rule Definition
Defines the name _assoc of an association defined using ASSOCIATION of a CDS view. If no name is defined explicitly using AS, _assoc is set implicitly to the name of the target data source. Association names should start with _.

Remediation
Association name should start with _.

Violation Code Sample

buyer name does not start with "_"
----------------------------------
define view sales_order_invoice_header as
  select from snwd_so_inv_head
         association [1..1] to snwd_bpa as buyer
           on $projection.buyer_guid = buyer.node_key
         { key snwd_so_inv_head.node_key,      //used in assoc _invoice_items
               snwd_so_inv_head.buyer_guid,    //used in assoc _buyer
               buyer.bp_id as buyer_id,       //from assoc _buyer
               snwd_so_inv_head.payment_status,
              @Semantics.currencyCode
               snwd_so_inv_head.currency_code,
              @Semantics.amount.currencyCode: 'currency_code'
               snwd_so_inv_head.gross_amount
         }
          where buyer.bp_role = '001';buyer name does not start with "_"

----------------------------------
Association has no name
----------------------------------
define view sales_order_invoice_header as
  select from snwd_so_inv_head
         association [1..1] to snwd_bpa
           on $projection.buyer_guid = buyer.node_key
         { key snwd_so_inv_head.node_key,      //used in assoc _invoice_items
               snwd_so_inv_head.buyer_guid,    //used in assoc _buyer
               buyer.bp_id as buyer_id,       //from assoc _buyer
               snwd_so_inv_head.payment_status,
              @Semantics.currencyCode
               snwd_so_inv_head.currency_code,
              @Semantics.amount.currencyCode: 'currency_code'
               snwd_so_inv_head.gross_amount
         }

Fixed Code Sample

define view sales_order_invoice_header as
  select from snwd_so_inv_head
         association [1..1] to snwd_bpa as _buyer
           on $projection.buyer_guid = _buyer.node_key
         association [1..*] to snwd_so_inv_item as _invoice_items
           on $projection.node_key = _invoice_items.parent_key
         { key snwd_so_inv_head.node_key,      //used in assoc _invoice_items
               snwd_so_inv_head.buyer_guid,    //used in assoc _buyer
               _buyer.bp_id as buyer_id,       //from assoc _buyer
               snwd_so_inv_head.payment_status,
              @Semantics.currencyCode
               snwd_so_inv_head.currency_code,
              @Semantics.amount.currencyCode: 'currency_code'
               snwd_so_inv_head.gross_amount,
               _invoice_items                  //publish assoc _invoice_items
         }
          where _buyer.bp_role = '001';          //usage of assoc buyer

https://blogs.sap.com/2019/10/09/abap-cds-views-development-guidelines-and-naming-conventions/ https://help.sap.com/doc/abapdocu_752_index_htm/7.52/en-US/abencds_f1_association.htm