Update a field when receiving a 2nd line item

Question

I have a custom object. When receiving a second line item on that custom object I want a check box field to be updated to true.

I have tried using workflow rule but I couldn't find any relation for the 2nd line item.

Any suggestions? Maybe a trigger but how to lock the 1st line item and fire the trigger when the 2nd line item is in place

I am editing this to give more info. The custom object has a master detail relationship with Contacts. I then created a custom field on contacts(checkbox) where I want this to become true when a second record is on the custom object. I have then created a roll up field that counts that custom object's records and created a workflow rule where rollupfield>1 and the action was a field update of my checkbox to true, BUT still getting unchecked button

Answer 1

If you are using a rollup field to count the related objects I would use a formula field checkbox on the Contact that is true when the rollup field is greater than 1.

Answer 2

So after a help from Geoffrey Flynn,

1)Roll up field on the master object to count the records. Filter on the rollup field to get the status. 2) Workflow on the master object with evaluation at created and everytime edited and a formula where rollupfield>1 3) Then a field update action where you update the checkbox to true