Queries on creating relationship between tables

Hi All,

Below is the code of 1:N relationship between 2 tables

<element label="Dealers" name="dealer" revLink="vehicleLink" target="all:dealer"  type="link">

      <join xpath-dst="@vin" xpath-src="@dealerId"/>

  </element>

1.In the above code @vin,@dealerId are not any key they are just attributes from two different tables. Can relationship can be done using attributes instead of PK in <join> ?

2. all:dealer is a custom schema created for global market. For mexico market we have extended all:dealer with nmx namespace.When creating relationship between vehicle links and dealer  tables in target which schema should I mention all:dealer or nmx:dealer ?

3 .all:dealer has autopk. In the extension schema nmx:dealer can we create a custom attribute "dealerId" as primary key. In this case what will be the impact if the table all:dealer has two primary key(autopk "id" and dealerId as custom PK) .Is it a good practice to have custom primary key for extension schema .Will this have any impact on all:dealer  table

Thanks,

Maheswari