ERROR 1452: Cannot add or update a parent row: a foreign key code example

Example 1: error 1451 (23000): cannot delete or update a parent row: a foreign key constraint fails

SET FOREIGN_KEY_CHECKS = 0; -- to disable FK checks (ex: for delete purpose)
SET FOREIGN_KEY_CHECKS = 1; -- to re-enable FK checks

Example 2: integrity constraint violation: 1452 cannot add or update a child row: a foreign key constraint fails

It just simply means that the value for column project_id on table comments you are inserting doesn't exist on table projects. Bear in mind that the values of column project_id on table comments is dependent on the values of ID on table Projects.

The value 50dc845a-83e4-4db3-8705-5432ae7aaee3 you are inserting for column project_id does not exist on table projects.