contract/product_contract/views/sale_order.xml

<?xml version="1.0" encoding="utf-8"?>

<!--
    Copyright 2018 ACSONE SA/NV.
    License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
-->

<odoo>

    <record id="view_order_form" model="ir.ui.view">
        <field name="name">sale.order.form (in product_contract)</field>
        <field name="model">sale.order</field>
        <field name="inherit_id" ref="sale.view_order_form"/>
        <field name="arch" type="xml">
            <xpath expr="//div[@name='button_box']" position="inside">
                <button name="action_show_contracts"
                        type="object" icon="fa-book"
                        class="oe_stat_button"
                        attrs="{'invisible': ['|', ('is_contract', '!=', True), ('state', 'not in', ['sale', 'done'])]}">
                    <field string="Contracts"
                           name="contract_count"
                           widget="statinfo"/>
                </button>
            </xpath>
            <xpath expr="//field[@name='order_line']" position="before">
                <field name="is_contract" invisible="1"/>
            </xpath>
            <xpath expr="//field[@name='order_line']/form//field[@name='product_id']"
                   position="after">
                <field name="contract_template_id" invisible="1"/>
                <field name="contract_id"
                       options='{"no_create": True}'
                       attrs="{'invisible': [('is_contract', '=', False)]}"
                       domain="['|',('contract_template_id','=',contract_template_id),
                       ('contract_template_id','=',False),
                       ('partner_id','=',parent.partner_id),
                       ('recurring_invoices','=',True),
                       ]"/>
                <field name="contract_line_id"
                       attrs="{'invisible': [('is_contract', '=', False)]}"
                       domain="[('contract_id','=',contract_id)]"/>


            </xpath>
            <xpath expr="//field[@name='order_line']/form//field[@name='tax_id']/parent::group"
                   position="after">
                <field name="is_contract" invisible="1"/>
                <separator colspan="4" string="Recurrence Invoicing"
                           attrs="{'invisible': [('is_contract', '=', False)]}"/>

                <group attrs="{'invisible': [('is_contract', '=', False)]}">
                    <field name="recurring_rule_type"/>
                </group>
                <group attrs="{'invisible': [('is_contract', '=', False)]}">
                    <field name="recurring_invoicing_type"
                           attrs="{'invisible': [('recurring_rule_type', '=', 'monthlylastday')]}"/>
                </group>
                <group attrs="{'invisible': [('is_contract', '=', False)]}">
                    <field name="date_start"
                           attrs="{'required': [('is_contract', '=', True)]}"/>
                </group>
                <group attrs="{'invisible': [('is_contract', '=', False)]}">
                    <field name="date_end" attrs="{'required': [('is_contract', '=', True)]}"/>
                </group>
            </xpath>
            <xpath expr="//field[@name='order_line']/tree//field[@name='price_total']"
                   position="after">
                <field name="date_start"
                       attrs="{'column_invisible': [('parent.is_contract', '=', False)]}"/>
                <field name="date_end"
                       attrs="{'column_invisible': [('parent.is_contract', '=', False)]}"/>
            </xpath>
            <xpath expr="//field[@name='order_line']/tree"
                   position="attributes">
                <attribute name="editable"/>
            </xpath>
        </field>
    </record>

</odoo>