[Solved] Simple explanation of one2many, many2one...

kilrou · **Joined:** Mon May 16, 2011 4:20 pm **Posts:** 27

Hello,

as the subject suggests, would you have a simple explanation of these three fields, it's difficult to capture their interest and in which situation to use one over the other.

Thank you in advance.

hummingbird · **Posted:** Wed Aug 03, 2011 12:47 am

one2many:
invoice->invoice_line
An invoice has many lines.
In the database the lines each have a reference to their invoice.

many2one:
invoice->partner
Many invoices belong to a partner.
In the database the invoices each have a reference to their partner.

many2many:
users<->groups
A user belongs to many groups and a group has many users.
Or
A user has many groups and a group belongs to many users.
It works both ways.
In the database a separate table has a user-group reference pair for each relation.

gdaddy · **Posted:** Wed Aug 03, 2011 1:55 am

The explanation is correct in simple terms and common understanding, but in relational database terms it is more like

one2many
An invoice has 0,1 or many lines.
Note this is an important distinction as the one2one field type has been deprecated.

many2many
A user belongs to 0,1 or many groups and a group has 0,1, or many users.

kilrou · **Joined:** Mon May 16, 2011 4:20 pm **Posts:** 27

Thanks for your good explanations.

Best regards.