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How to compare Trigger.new values with Trigger.old values
I have a junction object ‘Related Products’ which has master detail relationship with ‘Contact’ and have a lookup relationship with ‘Product’. And I have added a related list of ‘Related Products’ in Contact detail page where user can create new Related Product from the Contact and can add multiple Products to it.
Now my requirment is “if any user selects a product which is already selected to that contact should through error”.
For example, I have contact called “Test Contact111”, In it I have created one Related Product “RP-0001” and selected a Product called “Prod111”, It will get to save and will show in the Contact’s detail page under ‘Related Products’ section.
Again from the same Contact I will create another Related Product “RP-0002” and again will select the Same Product “Prod111”, now the trigger need to throw an error states “Prod111 is already selected”
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