Can Appwrite Relation reference to a unique key in other table?

I have used SQL databases. There is a concept of a foreign key, referencing another unique key in another table. I have seen that relationships are used in Appwrite. What I have understood is that the relationship references the primary key in another table. Is there a way for a column in my table to reference another unique key in another table, i.e., creating a foreign key? Thank You.