project.py

from odoo import (
    models,
    fields,
    )


class ProjectProduct(models.Model):
    _name = 'project.product'
    _description = 'Product on project'

    project_id = fields.Many2one(
            'project.project', required=True, ondelete='cascade')
    product_id = fields.Many2one('product.product', required=True)


class Project(models.Model):
    _inherit = 'project.project'

    product_ids = fields.One2many('project.product', 'project_id')


class Task(models.Model):
    _inherit = 'project.task'

    product_id = fields.Many2one('product.product')