project question

We were trying to workon the forms that will allow us to input data online. However, we constantly run into this error message that reads "cannot insert NULL into ("NLAPOULA"."CONTACT"."CONTACT_ID") ORA-06512: at line 2" Contact_id and donor_id are supposed to be automatically generated. Contact_id is the primary key where the donor_id is the foreign key. I feel like the problem resides in the donor_id. What do we need to do to avoid this error and at the same time allow donor_id to be automated? We are currently using <td><xsl:value-of select="donor_id"/></td to automate this.