In standard Rails applications, the default is opposite: belongs_to adds an automatic validation to foreign key columns enforcing the presence of a valid value unless you explicitly add the option optional: true.

So remember — if you’re ever in the situation of:

-you’re trying to save a record

-the save is failing

-you’re doing the standard debugging technique of printing out zebra.errors.full_messages

-you’re seeing an inexplicable validation error message saying that a foreign key column is blank

-now you know where the validation is coming from: belongs_to adds it automatically

-so figure out why you’re not providing a valid foreign key (usually it is because the parent object failed to save for its own validation reasons)