HI Eric,

The issue suspected from your code is if your function method is call for only one field.

Than you can directly assigned return value to ID, you don't need to pass value with field. If your method is called for any new records means this is the fault.

Currently you did this.

 res.update({rec.id:{ field_name: None }}) 

Instead try this

res[rec.id] = your_val or None  

Or if your method is not called for already calculated record it means its store=true issue.

Here is the explanation of store=true..

When you define any functional field. than you will have

these possibilities :

when you set store=True it once the value of this will calculated will be stored in database and so ID of that record will not pass in list to recalculate if it is already calculated.

when  you haven't applied store=True than all the that ID will be pass to recalculate if any changes made on that record.

or

If you want to reconsecrate your information after storing in database than you can use store = {} (dictionary option)

Here in dictionary you pass the number of model and fields based on that your fields values is going to update and it will store as well.

The real example of store you will find in account module with account.invoice model. functional field with store= {}.

Any one who knows more about functional field than can improve my Answer.

Hope this will help.

Rgds,

Anil.