Barefoot learning LINQ (033): the two entity classes that establish mappings must contain a primary key column Member respectively.

Http://u.115.com/file/f2cb57bffb demo

Demonstration highlights
The two entity classes that establish the ing relationship must respectively contain a primary key column member, such as the primary key attribute or primary key field.
Under normal circumstances, the primary key column members should be the primary key columns mapped to the data table.
If you change the primary key column members of the two object classes to normal members in the object link designer, although the dbml file still retains the ing relationship, however, the object link designer does not generate code for ing.
In addition, only one primary key column member is required for the two entity classes to establish the ing relationship. It is not mandatory that these primary key column members do not map to the primary key column of the data table.