Comma-separated related objects to return in full instead of as IDs. Accepted values: contact, contact.persons, employee, employee.reimbursements_account, journal, paid_through_account, paid_through_account.parent, payment_fees_account, payment_fees_account.parent, project, payment_request, payment_request.attachments, payment_request.bank_account, payment_request.bills, payment_request.contact, payment_request.cost_center, cost_center. Nested relations are joined with a dot. Pass journal to replace the journal ID with its object (ID, date, currency, reference, notes and timestamps). Documents without a journal still return null. Journal line items are retrieved separately with GET /v1/journal-line-items/?journal={journal_id}.