Relationships - Many2one, One2many, Many2many

Why Relationships Are Critical

Relationships connect data across different models. Understanding them is important for:

• Understanding how data flows in Odoo
• Creating proper filters and reports
• Designing new features correctly

The Three Relationship Types

graph TD
    M2O["Many2one
    N : 1"] -->|Many orders belong to One customer| C[Customer]
    O2M["One2many
    1 : N"] -->|One customer has Many orders| O[Orders]
    M2M["Many2many
    N : N"] ---|Many products in Many categories| CAT[Categories]

Type	Cardinality	UI Display	Database Storage
Many2one	Many → One	Dropdown	Foreign key ID in column
One2many	One → Many	Embedded list	Not stored (virtual)
Many2many	Many ↔ Many	Tags/multi-select	Junction table

Many2one - The Dropdown Link

What Is Many2one?

A Many2one field creates a link from many records in one model to one record in another model.

• UI: Dropdown / Search field
• Storage: Just an ID number (foreign key)
• Example: Many Sale Orders → One Customer

Real-World Example: Sales Orders

graph LR
    S1[Sale Order #1] --> C[Customer: Acme Corp]
    S2[Sale Order #2] --> C
    S3[Sale Order #3] --> C
    style C fill:#10b981,color:white

Many Sale Orders can belong to One Customer.

Common Many2one Fields

Field	On Model	Links To	Business Meaning
partner_id	sale.order	res.partner	Which customer is this order for?
product_id	sale.order.line	product.product	Which product is on this line?
company_id	(many models)	res.company	Which company owns this record?
user_id	sale.order	res.users	Who is the salesperson?
department_id	hr.employee	hr.department	Which department?
project_id	project.task	project.project	Which project does this task belong to?

Many2one Field Attributes

Attribute	Purpose	Example
comodel_name	Target model	'res.partner'
ondelete	What happens when target is deleted	'restrict', 'cascade', 'set null'
domain	Filter available options	[('is_company', '=', True)]
context	Pass context to linked views	{'default_type': 'out_invoice'}

One2many - The Reverse View

What Is One2many?

A One2many field shows all records that link back to the current record via a Many2one.

• Important: One2many fields are NOT stored in the database. They are computed by looking at the reverse relationship.
• UI: Embedded list/table (e.g., order lines within an order)

Real-World Example: Customer's Orders

On the Customer form, you see a list of all their orders:

graph TD
    C[Customer: Acme Corp] -->|order_ids| List
    subgraph List [One2many List]
        O1[Sale Order #1]
        O2[Sale Order #2]
        O3[Sale Order #3]
    end
    style C fill:#10b981,color:white

The customer doesn't "store" these orders - Odoo looks up "all orders where partner_id = this customer"

One2many Field Attributes

Attribute	Purpose	Example
comodel_name	Target model (the child)	'sale.order.line'
inverse_name	Many2one field on child pointing back	'order_id'
domain	Filter which records to show	[('state', '=', 'active')]
copy	Copy lines when duplicating parent	True or False

One2many Limitations

Because One2many is not stored in the database, you cannot:

• Use it in "Group By" reports
• Filter by it directly in domains (some limitations)
• Export it directly to Excel (export from child model instead)

Many2many - Multiple Links Both Ways

What Is Many2many?

A Many2many field allows linking multiple records on both sides.

• Storage: Creates a "junction table" in the database
• UI: Tags or multi-select dropdown

Real-World Example

Products ↔ Categories

Product	Categories
Laptop	Electronics, Computers, Office Equipment
Keyboard	Electronics, Office Supplies, Computer Accessories
Mouse	Electronics, Computer Accessories

The same product can be in multiple categories, and the same category can contain multiple products.

Common Many2many Fields

Field	On Model	Links To	Usage
tag_ids	sale.order	crm.tag	Categorizing orders
category_id	res.partner	res.partner.category	Customer tags
tax_ids	sale.order.line	account.tax	Multiple taxes on one line
user_ids	res.groups	res.users	Users in a security group

Many2many Field Attributes

Attribute	Purpose	Example
comodel_name	Target model	'res.partner.category'
relation	Junction table name	'partner_category_rel'
column1	Column for current model ID	'partner_id'
column2	Column for target model ID	'category_id'

Quick Reference: Relationship Summary

Type	Stored?	UI Display	Can Group By?	Example
Many2one	Yes (ID)	Dropdown	Yes	Order → Customer
One2many	No	Embedded List	No	Customer → Orders
Many2many	Yes (Junction)	Tags	Limited	Product ↔ Categories

Deletion Behavior (ondelete)

When you delete a record (e.g., a Customer), what happens to the linked records (Orders)?

Behavior	Effect	Example
Restrict	Blocks deletion (Error message)	Can't delete Customer if they have Invoices
Set Null	Clears the link (Field becomes empty)	Employee leaves; tasks stay but user_id cleared
Cascade	Deletes linked records	Delete an Order → All Order Lines deleted

"Cannot Delete" Errors

If you get a "Reference Error" preventing deletion, it's usually ondelete='restrict'. You must first archive, reassign, or delete the related records.

Real-World Analogy

Behavior	Analogy
Restrict	Can't close a bank if customers have money there
Set Null	Employee leaves company - their records stay but employer field is cleared
Cascade	Delete a department - all positions in it are also eliminated

Understanding Related Fields

Definition

A related field displays information from a linked record directly on the current form. It "reaches through" a Many2one to show data. Example: Showing the Customer's Country on the Sales Order.

Common Related Fields You See Daily

Visible On	Shows Field	Source Path
Sales Order	Customer's delivery address	partner_shipping_id.street
Invoice Line	Invoice Status (Draft/Posted)	move_id.state
Stock Move	Customer on the transfer	picking_id.partner_id
Employee	Manager Name	department_id.manager_id.name

Stored vs Not Stored Related Fields

Type	Behavior	Use Case
Not Stored (Default)	Always live data, can't group by	Display-only info
Stored	Copies value, can group by, might get "stale"	Reporting, filtering

Stale Data Risk

Stored related fields copy the value at the time it's computed. If the source changes without triggering recomputation, the related field may show outdated data.

Relationship Decision Guide

Business Need	Relationship Type	Example
Link to one other record (dropdown selection)	Many2one	Order → Customer
Show all linked items as a list	One2many	Customer form showing all their orders
Tag or categorize with multiple options	Many2many	Product ↔ Multiple categories
Display info from linked record (read-only)	Related field	Show customer's country on order

Common Business Patterns

Pattern 1: Document with Lines

Almost every business document in Odoo follows this structure:

Header (Parent)	Lines (Children)
Sales Order	Sales Order Lines
Purchase Order	Purchase Order Lines
Invoice (account.move)	Invoice Lines (account.move.line)
Delivery Order (stock.picking)	Stock Moves (stock.move)

The header "owns" the lines. Delete an order → lines are deleted too (cascade).

Pattern 2: Hierarchies (Self-References)

Some models have parent-child relationships within themselves:

Model	Hierarchy Example
Product Categories	Electronics > Computers > Laptops
Departments	Sales > Europe > France
Chart of Accounts	Assets > Current Assets > Bank
Locations	Warehouse > Shelf A > Bin 1

These use a parent_id Many2one field pointing to the same model.

Pattern 3: Tagging / Categorization

When something can belong to multiple categories simultaneously:

Model	Tag Field	Usage
CRM Leads	tag_ids	Hot Lead, VIP, Trade Show
Products	product_tag_ids	Multiple website categories
Contacts	category_id	Supplier, Reseller, Partner

These use Many2many relationships displayed as colored tags in the UI.

Product Template vs Product Variant

Confusing Concepts - Must Understand!

product.template and product.product are two different models!

Model	Definition	Example
product.template	The Master (Shared info)	"T-Shirt" with name, description, category
product.product	The Variant (Specific item)	"T-Shirt, Red, Size M" with unique barcode

Key insight: Sales orders use product.product (variant), but the product form edits product.template.

Similarly: Users and Partners

res.users (login accounts) "inherits" from res.partner (contacts). Every user IS a contact with additional login capabilities:

• Creating a user automatically creates a contact
• User's name, email, phone come from their partner record
• You can message a user through the contact system

Why Can't I Group By Some Fields?

One2many Fields Are Virtual

One2many fields don't actually exist in the database - they're calculated on-the-fly. This means:

• Cannot Group By: "Group orders by order lines" doesn't make sense
• Cannot Filter Directly: Use the child model instead
• Cannot Export: Export from the line model, not the parent

Workaround: Filter Through Child Model

Instead of trying to filter orders by product on the order line:

• Wrong approach: Search orders where order_line contains product X
• Right approach: Search order lines for product X, then look at their orders

In the UI, use the order line list view and group by Order to see the same data.

Knowledge Check

Q1: Sales Order to Order Lines - what relationship?

Answer: One2many on Order, Many2one on Line

The line stores the order_id (Many2one), and the order has a One2many pointing back.

Q2: Which relationship creates an actual column in the database?

Answer: Many2one

Many2one stores the foreign key ID in the table. One2many is virtual. Many2many uses a separate junction table.

Q3: Products and Tags - what relationship?

Answer: Many2many

Multiple products can have multiple tags. Odoo creates a junction table to store the connections.

Q4: What happens when you delete a customer with orders if ondelete='restrict'?

Answer: Deletion is blocked with an error message

Restrict prevents deletion and shows an error. You must first delete/archive the orders.

Q5: Why can't you Group By a One2many field?

Answer: One2many fields are not stored in the database

They're computed on-the-fly by looking at the reverse Many2one relationship. Since there's no column, there's nothing to group by.

Q6: What's the difference between product.template and product.product?

Answer: Template is the master, Product is the variant

product.template stores shared info (name, description). product.product stores variant-specific info (barcode, specific attributes). Sales orders use the variant.