Odoo, a powerful and versatile ERP system, uses various relational fields to manage data connections between models. One such relational field is one2many, which plays a crucial role in representing one-to-many relationships. This article will explain the concept of one2many fields in Odoo and illustrate their use with a real-world example.

What is a One2Many Relationship?

A one2many relationship in Odoo signifies that a single record in one model can be linked to multiple records in another model. This is particularly useful for managing hierarchical data structures, such as orders and their corresponding order lines, invoices and invoice items, or projects and tasks.

Defining One2Many Relationships in Odoo
To define a one2many relationship, you need two models:

A model that will hold the single record (the "parent" model).
A model that will hold the multiple related records (the "child" model).
The one2many field is declared on the parent model, while a corresponding many2one field is declared on the child model to create a bidirectional link.

Real Use Case Example: Sales Order and Order Lines
Let’s consider a common business scenario where a sales order can contain multiple order lines. Each order line represents a product or service being sold as part of the sales order.

Step-by-Step Implementation

1. Define the Parent Model (sale.order) First, we define the sale.order model, which represents the sales order.

from odoo import models, fields

class SaleOrder(models.Model):
    _name = 'sale.order'
    _description = 'Sales Order'

    name = fields.Char(string='Order Reference', required=True)
    date_order = fields.Datetime(string='Order Date', required=True, default=fields.Datetime.now)
    customer_id = fields.Many2one('res.partner', string='Customer', required=True)
    order_line_ids = fields.One2many('sale.order.line', 'order_id', string='Order Lines')

In this model:

name stores the order reference.
date_order records the order date.
customer_id links to the customer (partner) placing the order.
order_line_ids is a one2many field that links to the sale.order.line model.

1. Define the Child Model (sale.order.line) Next, we define the sale.order.line model, which represents the individual order lines within a sales order.

from odoo import models, fields

class SaleOrderLine(models.Model):
    _name = 'sale.order.line'
    _description = 'Sales Order Line'

    product_id = fields.Many2one('product.product', string='Product', required=True)
    quantity = fields.Float(string='Quantity', required=True)
    price_unit = fields.Float(string='Unit Price', required=True)
    order_id = fields.Many2one('sale.order', string='Order Reference', ondelete='cascade')

In this model:

product_id links to the product being ordered.
quantity specifies the quantity of the product.
price_unit records the unit price of the product.
order_id is a many2one field that links back to the sale.order model, creating the bidirectional relationship.

How It Works in Practice
When a user creates a new sales order in Odoo:

They fill out the main sales order details, such as the order reference, date, and customer.
Within the sales order form, they can add multiple order lines. Each order line specifies a product, its quantity, and the unit price.
The order_line_ids field in the sale.order model aggregates all related order lines, while each order line uses the order_id field to reference back to its parent sales order.
Benefits of Using One2Many Relationships
Data Organization: Structuring data in a hierarchical manner allows for better organization and retrieval.
Ease of Use: Users can easily manage related records within the parent record’s form view, improving usability and efficiency.
Data Integrity: Bidirectional relationships ensure that each child record is correctly linked to its parent, maintaining data consistency.
Conclusion
The one2many field in Odoo is a powerful feature for managing hierarchical data relationships. By understanding and utilizing this field type, you can create robust data models that reflect real-world business scenarios. The example of sales orders and order lines demonstrates how one2many relationships can be implemented to manage complex data structures effectively.

Using one2many relationships not only enhances data organization but also simplifies data management, making it easier for users to interact with and maintain the system. As you continue to develop and customize Odoo applications, leveraging one2many fields will be an essential tool in your arsenal.