How to truncate a foreign key constrained table in mysql?

You cannot TRUNCATE a table that has FK constraints applied on it (TRUNCATE is not the same as DELETE).
To work around this, use either of these solutions. Both present risks of damaging the data integrity.

Option 1:
Remove constraints
Perform TRUNCATE
Delete manually the rows that now have references to nowhere
Create constraints

Option 2:

SET FOREIGN_KEY_CHECKS = 0;
TRUNCATE table $table_name;
SET FOREIGN_KEY_CHECKS = 1;