1. In this recipe, we add a report on res.partner, which prints a list of books the partner borrowed. We need to add a one2many field on the partner model with relation to the model library.book.rent, as shown in the following example:

class ResPartner(models.Model):
    _inherit = 'res.partner'

    rent_ids = fields.One2many('library.book.rent', 'borrower_id')

2. Define a view for your report in reports/book_rent_templates.xml, as follows:

<?xml version="1.0" encoding="utf-8"?>
<odoo>
<template id="book_rents_template">
    <t t-call="web.html_container">
        <t t-foreach="docs" t-as="doc">
            <t t-call="web.internal_layout">
                <div class="page">
                    <h1>Book Rent for <t t-esc="doc.name"/></h1>
                    <table class="table table-condensed">
                        <thead>
                            <tr>
                                <th>Title</th>
                                <th>Expected return date</th>
                            </tr>
                        </thead>
                        <tbody>
                            <tr t-foreach="doc.rent_ids" t-as="rent" >
                                <td><t t-esc="rent.book_id.name" /></td>
                                <td><t t-esc="rent.return_date" /></td>
                            </tr>
                        </tbody>
                    </table>
                </div>
            </t>
        </t>
    </t>
</template>
</odoo>

3. Use this view in a report tag in reports/book_loan_report.xml, as shown in the following example:

<?xml version="1.0" encoding="utf-8"?>
<odoo>
    <report id="report_book_rent"
        name="my_library.book_rents_template"
        model="res.partner"
        string="Book Rents"
        report_type="qweb-pdf" />
</odoo>

4. Add both files in the manifest of the add-on and add contacts in depends, so you can open the form view of the partner, as shown in the following example:

...
    'depends': ['base', 'contacts'],
    'data': [
        'views/library_book.xml',
        'views/library_member.xml',
        ...
        'reports/book_loan_report.xml',
        'reports/book_loan_report_template.xml',
    ],
...

Now, when opening the partner form view, or when selecting partners in the list view, you should be offered the option to print the book loans in a drop-down menu, as shown in the following image: