This means you are using a hibernate mappings like
Now for xml configuration you should include
cascade="all" or for class based annotations
cascade=CascadeType.ALL on your collection mapping.
This happens because you have a collection in your entity which has some elements which are not persisted in the database yet and hence it gives the error.
By mentioning the cascading types in your entity you tell hibernate to save all the child entities on saving the parent entity.