Restrict user to add "3rd" entry if capacity is 2

Question

Hi, 
 
 I have 2 tables pen and animal table , when i am adding the animal i am assigning the pen to the animal and the pen name are coming from pen table, 
 i want to restrict user to add "3rd" animal if i have already set the capacity of pen to 2 animals only in pen table , any suggestions? 
 
 while adding animal user should get the alert mst or validation like " pen have capacity of 2 animals only select diff pen".

Mathieu Drouin · Answer

You'll have to do an expression rule that queries the database (query entity or record type) and checks the number of animals for a particular pen. You can then use that query in the validations attribute of the Pen name field.