Problem

You want to create both a main record and its linked relation record in one form view, without merging the models.

Your current XML tries to embed a <form> inside a <field>, which isn't valid Odoo syntax.

Correct Approach

Use inline editing with the options attribute on the many2one field. Here’s how:

1. Update Your XML View

xml

<record id="view_main_form" model="ir.ui.view">
    <field name="name">main.form.view</field>
    <field name="model">main</field>
    <field name="arch" type="xml">
        <form>
            <sheet>
                <group>
                    <field name="name"/>
                </group>
                <notebook>
                    <page string="Relation">
                        <field name="relation_id" 
                               options="{'no_open': True, 'no_create': False}"
                               context="{'form_view_ref': 'your_module.view_relation_form'}"/>
                    </page>
                </notebook>
            </sheet>
        </form>
    </field>
</record>

2. Create a Separate Form View for relation

Define a simplified form for quick creation:

xml

<record id="view_relation_form" model="ir.ui.view">
    <field name="name">relation.popup.form</field>
    <field name="model">relation</field>
    <field name="arch" type="xml">
        <form>
            <group>
                <field name="name_relation"/>
                <field name="second_field"/>
            </group>
        </form>
    </field>
</record>

3. Key Attributes Explained

• options="{'no_open': True}": Prevents opening the full record view.
• no_create: Set to False to allow new record creation.
• context="{'form_view_ref': ...}": Forces the popup to use your custom form.

Alternative: Default Values

If you always want to auto-create a relation when creating a main record:

1. Override create in main model

python

@api.model
def create(self, vals):
    if not vals.get('relation_id'):
        relation = self.env['relation'].create({
            'name_relation': vals.get('name') + " Relation",
            'second_field': "Default Value",
        })
        vals['relation_id'] = relation.id
    return super().create(vals)

Result

Now when you:

1. Create a new main record,
2. Click the relation_id field,
3. Select "Create and Edit",
4. A popup with your custom relation form appears.

After saving both forms, the records are linked automatically.

Why This Works

• Odoo’s many2one field natively supports inline creation.
• The options and context attributes control the behavior.
• No need for nested <form> tags (which aren’t supported).
  
  

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(Need further customization? Drop a comment—I’m happy to refine the solution!)

In Odoo, a Many2one field by default shows as a dropdown list with a search option. However, when we define a nested <form> inside a <field name="relation_id"> (like in your example), Odoo shows the linked record in a form view, allowing users to create or edit the related record directly from the main form.

When you open the Many2one field in the UI, Odoo shows a mini form with all the fields of the related model.