diff -r 989bbadbcd8d -r aaabcb64f77f server/schema2sql.py
--- a/server/schema2sql.py	Thu Jan 15 16:49:32 2015 +0100
+++ b/server/schema2sql.py	Thu Jan 15 17:06:16 2015 +0100
@@ -187,8 +187,8 @@
 
 _SQL_SCHEMA = """
 CREATE TABLE %(table)s (
-  eid_from INTEGER NOT NULL,
-  eid_to INTEGER NOT NULL,
+  eid_from INTEGER NOT NULL REFERENCES entities (eid),
+  eid_to INTEGER NOT NULL REFERENCES entities (eid),
   CONSTRAINT %(table)s_p_key PRIMARY KEY(eid_from, eid_to)
 );