# Extend model

Sometimes we want to extend an existing `Model` to add `Field` or methods. This can be done using the extension mechanism of Tryton which can combine classes with the same `__name__` that are registered in the `Pool`.

## Extend the Party model

Let’s add an `opportunities` field on the `party.party` model. The model in `party.py` file of our module looks like this:

    from trytond.model import fields
    from trytond.pool import PoolMeta

    class Party(metaclass=PoolMeta):
        __name__ = 'party.party'
        opportunities = fields.One2Many(
            'training.opportunity', 'party', "Opportunities")

This new class must be register in the `Pool`. So in `__init__.py` we add:

    from . import party

    def register():
        Pool.register(
            ...,
            party.Party,
            module='opportunity', type_='model')

## Extend the Party view

Now that we added a new field to the `party.party` `Model`, we can also add it the form view. This is done by adding a `ir.ui.view` record that inherit the party form view of the `party` module. Here is the content of the `party.xml` file:

    <tryton>
       <data>
          <record model="ir.ui.view" id="party_view_form">
             <field name="model">party.party</field>
             <field name="inherit" ref="party.party_view_form"/>
             <field name="name">party_form</field>
          </record>
       </data>
    </tryton>

The `type` is replaced by:

`inherit`

  A reference to the XML id of the view extended prefixed by the name of the module where the view is declared.

The content of the inheriting view must contain an XPath expression to define the position from which to include the partial view XML. Here is the content of the form view in `view/party_form.xml`:

    <data>
       <xpath expr="/form/notebook/page[@name='identifiers']" position="after">
          <page name="opportunities" col="1">
             <field name="opportunities"/>
          </page>
       </xpath>
    </data>

And finally we must declare the new XML data in the `tryton.cfg` file:

    [tryton]
    ...
    xml:
       ...
       party.xml

## Update database

As we have defined new field and XML record, we need to update the database with:

    $ trytond-admin -d test --all

And restart the server and reconnect with the client to see the new field on the party:

    $ trytond

Let’s use a wizard to convert the opportunity.