Line item id value

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I'm hoping to understand how the id value of a line item is generated. As far as I can tell it is commonly an integer, which would make sense as an id field. However, I've noticed it is possible for the id to be an alphanumeric string. My understanding (from various other support articles) is that the string is a result of an MD5 hash of some of the line item values. I'm struggling to recreate the string id value in my store. Previously my app was only accepting integers as the line item id, which caused errors with these strings. I've removed the type hinting, but would like to test my changes using a real Shopify line item with a string id. 

Is anyone aware of a way to generate a line item object that has a string id?