Validate Notes in Child Entity

Hi All,

I have requirement on click of button i need to makesure that notes have been added in the child entity .If the attachment is not added then user should get an alert stating notes is not attached.

Opportunity is the primary entity

Documents(For custom notes requirement )custom entity i an adding notes in this custom entity.

I have added subgrid (for attaching notes in Documents entity )in Opportunity.

Now i need to validate notes is getting attached if not alert the user.

Let me know if any suggestions