How can I query the AppEngine database by comparing two properties of the same object?

Question

I would like to make a request to return all entities of a given type where one property is larger than another. For example, return all widgets where the sale price is greater than the purchase price. I cannot figure out how to do this in the Google AppEngine datastore.

Answer 1

You cannot do this directly, because the indexing model does not support complex expressions. You will need to add a new property that you manually maintain, like the difference between the purchase and sale prices, and then look for elements where this delta property is greater or less than zero.