Using Tabular Models - SQL Server Analysis Services 2012 Cube Development - page 316

6. The third model modification is adding relationships. Relationship building is
likely the most important part of the modeling process as it allows the user
to interact with the data in interesting ways. However, you should fix columns
and data types prior to working with the relationships.
7. In SSDT, you should look for the diagram view. This is the easiest way to
visualize the data relationships. You can get to the relationship view either by
accessing the main menu or a small button on the design surface. Navigate
to the menu option via MODEL | Model View | Diagram View . The button on
the design surface is at the lower right-hand corner on your Model.bim tab.
Refer to the following screenshot to see what to look for: