I am using Odoo 8 with the following modules:

1) tko_account_contract_report_template
this allows me to print a report containing the contract content, including a predefined signature.

2)chricar_partner_id_number
this allows me to associate ID numbers such as driver's license, health card, Passport number, etc. to each partner.

From within the contract report template I am able to reference fields that are directly associated with the contract such as  

Name: $(partner_id.name)s and Email: $(partner_id.email). These work as expected, no problems at all.

What I would like to be able to do, but don't know how, is to reference other fields in the res.partner.id_category and res_partner_id_number, both fields created by the chricar_partner_id_number module.I am familiar with SQL query language using the JOIN statement to bring together data from various related tables, but do not understand how it works in Odoo.

Any help would be much appreciated.