I am using a databse first method. I generated edmx from the database. But diagrams for the tables which are in many to many relationships do not display in the generated edmx.
If your Many to many lookup table consists of just Foreign Keys and no other fields, then it will not show up in your edmx file as a Table. It will show up under Navigation Properties of each table though.
来源：https://stackoverflow.com/questions/7544259/the-diagram-for-the-many-to-many-relationship-table-does-not-appear-in-the-edmx