Compute/auto-manipulate values stored in Entity Attributes (in Service Studio)

I would like to know how (in which ways) I can auto-manipulate (compute) values that are stored in existing Entity Attributes (working in Service Studio). Consider this example:

The Entity 'John' stores the Entity Attribute 'Age' set to [20]. (data is 'public' and not 'expose read only')

A button on my Webpage should compute [20 + 1] when pressed and update John's Age.

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The way I put my logic I can publish but get an error on my webpage when using the button. In red it says: [ISE1JM018].DBO.[OSUSR_Z7I_JOHN] with key 0 was not updated.

I currently use a screen action (where the button links to). I simply cannot figure out how this is supposed to be done correctly by the Outsystems Logic Elements. It seems 'Assign' can only be used for variables in the current scope, so I use an 'Aggregate' before assigning. What has to be written in the in the Assign's 'Assignments'? Or must I use Integration Studio instead?

Sadly my screenshot is not displayed :-/
Thanks in advance.

I think you didn't add a filter to the Aggregate; that queries on the Form.Id; so the UpdateJohn tries to update the record with key 0. (and this record is not available, like the error explains) 

Hope this helps...


Maybe not 100% on topic, would it not be smarter to store the birthdate in the entity and use the GetAge() function to calculate the age from it.?