Hello Tarun,

Based on your description, you're encountering a common Studio limitation with relational data persistence. Here's how to make this work without backend code:

The Root Problem

When using Studio-created models with One2many relationships, Odoo often fails to properly save the relational data due to missing inverse fields in the underlying database structure.

Working Solution (Pure Studio Approach)

1. Recreate Your Models Correctly

For Participants:

1. In Studio, create "Event Participant" model
2. Critical Fields:
   • event_id (Many2one to Event)
   • partner_id (Many2one to res.partner)
   • status (Selection field)
   • ticket_type (Char or Many2one to a custom "Ticket Type" model)

For Vendors:

1. Create "Event Vendor" model
2. Critical Fields:
   • event_id (Many2one to Event)
   • partner_id (Many2one to res.partner)
   • expense_category (Many2one to a custom "Expense Category" model)

2. Configure the Event Form

1. Edit the Event form in Studio
2. Add two new tabs
3. For each tab:
   • Add a One2many field
   • Set the relation to your custom model
   • Set the inverse field (event_id)

3. Fixing the Disappearing Data Issue

The key is to use the form view widget instead of inline:

1. Remove your current One2many fields
2. Re-add them but:
   • Click the gear icon on the One2many field
   • Change Widget from "Many2many" to "One2many"
   • Set View Mode to "form,tree" (not just "tree")
   • Enable "Allow creation", "Allow deletion"

4. Alternative Approach Using Existing Relations

Instead of custom models, leverage Odoo's built-in:

• Participants: Use event.registration model (already exists)
• Vendors: Use event.sponsor model (exists in Enterprise)

You can customize these existing models in Studio without creating new ones.

Why This Works

• Using form view forces proper record creation before linking
• Built-in models already have proper database relations
• Odoo handles the inverse field management automatically

Limitations to Accept

1. Email Automation: Studio can't create automated emails based on these custom tabs
2. Advanced Filtering: Complex domain filters may not work
3. Mobile App: Custom tabs may not appear in mobile views

Workaround for Email Notifications

Create manual email templates under Settings → Technical → Email → Templates, then:

1. Add a "Send Email" button to your custom tabs
2. Configure it to use your template
3. Users will need to click manually to send

Final Recommendation

For a production environment, consider:

1. Using Odoo Enterprise (has built-in sponsor/vendor features)
2. A one-time developer assist to properly create the models

Odoo.sh trial to test with more backend access

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