The things i sent and the things i receive share the same attributes  

Something like this?

Thanks

If one record only can have or a send user or a receive user, then you only need one foreign key in Trocas to User.

The send_receive does not need relationships to User, it's I'd should be a foreign key in Trocas to use tidy a record as sending or receiving.

Hi all, there´ s a previous post about this, on each exchange there´ s a sender and a receiver.  So 2 foreign keys in trocas, no need for that extra entity, i think.

But also spiritfox, i would make the pharmacy sender and receiver, not the user.

Dorine

Dorine, 

Thanks again for your help. 

I will simplify and create just one user. Each pharmacy will be a user