A way to reference the ID in a multi insert transaction? (postgres)
There are different ways to do this. 

The easiest way is to use the lastval() function which will return the value generated by the "last" sequence nextval.

START TRANSACTION;
INSERT INTO entity ...;
INSERT INTO t2 (eid, ...) VALUES (lastval(), ...), (...), (...);
COMMIT;


If you know the name of the sequence for the entity table you could also use the currval function:

START TRANSACTION;
INSERT INTO entity ...;
INSERT INTO t2 (eid, ...) VALUES (currval('entity_eid_seq'), ...), (...), (...);
COMMIT;


This can be written in  a more general way by using the pg_get_serial_sequence() function, avoiding to hardcode the sequence name:

START TRANSACTION;
INSERT INTO entity ...;
INSERT INTO t2 (eid, ...) VALUES (currval(pg_get_serial_sequence('entity', 'eid')), ...), (...);
COMMIT;


For more details, please see the manual: http://www.postgresql.org/docs/current/static/functions-sequence.html